So, before we go about making things move, we have to go through some very very basic stuff.

Gravity
All objects with mass have gravity. Gravity is a fundamental interaction between two objects that pulls the objects together. This interaction becomes weaker the further away you are from the other object.

Mass
Everything you slap onto your space/air thing has mass, be it a wing, fuselage, cockpit, parachute, anything.

Weight
Mass produces a force called weight when it's subjected to another object's gravitational field (In KSP's case, the planets and the star, Kerbol). This weight force pulls the object towards the center of the gravitational field (Basically the center of the planet/star).


Friction
If you've got some solid stuff moving forward, and that stuff is in contact with some other solid stuff (like the ground), it will produce a force that acts in the opposite direction of your direction of movement


Fluid
Fluids are basically liquids and gases, which, in the context of KSP, refer to water and the atmosphere respectively.

Drag
Basically friction, but instead of some solid stuff that is in contact with some other solid stuff, it's some solid stuff in contact with some other fluid stuff (Like air or water).


Lift
Lift is a force generated by an object moving through a fluid (like air or water). How it's generated is a complicated topic with people subscribing to many different theories so I'm not even gonna start explaining it here lest I get fan mail... Generally speaking, as you tilt a "wing" (Yes, your plane's fuselage can count as a wing if you want it to) back, it will generate lift and drag. Tilt it too far back and it'll stall (stops generating lift) because the air thinks you're asking too much from it and pisses off like my ex-girlfriend (its called flow separation if you wanna look it up. And yes, you can tilt it in the opposite direction, it'll still have the same effect, except the "lift" will be negative [downwards]).

Thrust
Thrust is a force usually produced by your engines by burning fuel/consuming electricity (for electric engines). It usually pushes in the opposite direction your engine is facing (propeller, abbreviated as prop, engines can be in tractor or pusher configuration. i.e., a 'pulling' prop and a 'pushing' prop).


Newton (N)
The Newton is the metric unit of force. Centimetres are to distance as newtons are to force.

Before we make things move, we have to talk about center of gravity (CoG), center of thrust(CoT), and center of lift (CoL).

CoG
CoG is an imaginary "balancing point" of an object.


CoT
Imagine the CoT as the point where the force produced by all your engines will act on (basically the summation of all the thrust produced by your engines)


CoL
Similar to the CoT, it's an imaginary point where all of your lift will act on (basically the summation of all the lift produced by your space/air thing)


So, now that we've cleared that up, how do we make things move?
It's simple, just apply a large enough force through the CoG of the thing in the direction you wish to move. If it overcomes any other forces, it'll move in the direction you want.


If force 1 is larger than force 2, the thing will move up.

How about rotating an object?
Simple, we apply moments onto an object to rotate it. What's a moment you ask, well, it's just a force applied at a distance from a specific point. The further away the force is from the specific point, the larger the moment you apply. (Moments = force x distance)

In the picture above, the triangle is the specific point that the seesaw will rotate around. The 3 newton force will produce a larger moment than the 5 newton force as the 3 newton force is further from the triangle, therefore, the seesaw will rotate clockwise, assuming it doesn't snap.

To rotate a flying space or air thing, take the CoG as the specific point where the moments will act about.

The force applied will cause the thing to turn counter clockwise.

So, now that we've learnt everything above, how do we make a space/air thing?

Space thing
Space things generally face forces like weight and drag. To make it into space, you need to get your space thing to overcome those forces by giving it enough thrust. Generally speaking, you don't want your space thing to rotate at launch, therefore, ensure that your CoT is inline with your CoG. You don't really have to worry about lift as your engines are the ones that will push you into space. Don't forget to add some RCS or SAS to control your space thing in space, as your wings won't work in space (There ain't a lot of fluid in space). Don't know where to put your RCS blocks? Use your newfound knowledge on moments to help you decide where to put them. Essentially, the further the force is from the CoG, the more moment it exerts, allowing it to turn faster.

Just some notes on RCS and SAS:
Reaction Control Systems (RCS) are tiny thrusters that consume monopropellant to produce small amounts of thrust.
Stability Augmentation Systems (SAS) are a form of autopilot that helps a space or air thing to dampen motion and hold a specific heading. It achieves this by using reaction wheels, flight control surfaces and RCS to dampen any unwanted motion, though sometimes it may fail to dampen the vehicle as the vehicle has inherent negative dynamic stability (basically it wobbles out of control).
Reaction wheels are basically spinning wheels that can turn a spacecraft as the wheels change speed. This uses the principle of the conservation of angular momentum.

Keep in mind that air isn't very abundant in space, so you'll need oxidizer for your engines to work as combustion requires oxygen and some fuel to burn, and you'll also need actual rocket engines instead of air breathing engines.

Also, and this is very important, CHECK YOUR STAGING. Make sure your various stages perform the right action, e.g. start engine > decouple > start another engine > decouple > parachute instead of decouple > start engine > parachute > start another engine > decouple

TL:DR, Just add enough engines, fuel and oxidizer.

But I wanna go into orbit!
That's easy, just add more fuel and oxidizer to make your space thing go sideways faster, and eventually, you'll reach orbit. As for going to other planets, same thing. Get enough fuel and oxidizer to push you into various orbits until you arrive at your destination. Think of this as pushing your craft upwards and, once you reach a decent altitude, sideways. This will push your craft in such a way where your craft will fall continuously but not hit the planet as it's moving sideways too fast.

Disclaimer
I don't have a lot of experience with space things and their engineering IRL, so there isn't much I can say about them.

Air thing
Air things face similar forces to space things, namely weight and drag. To make your air thing fly, you need to add enough thrust to push your air thing so that there will be a flow of air around your air thing's body. This will generate lift that will, well, lift your air thing into the air. When building your air thing, try to keep your CoG as close to the center of the air thing as possible, and try to keep your CoL slightly behind your CoG. It's also always best to keep your CoT inline with your CoG. Also, don't forget that fuel is consumed while your engines are on, so your CoG may change whilst your fuel gets consumed.

If the CoL is too far forward, it'll flip head over arse, too far back and it'll dive.

Controlling your air thing
There are three axes of rotation in flight; roll, pitch and yaw.


As discussed in the previous section, to make things rotate, you need to apply a force at a distance away from the CoG. The further away, the better. Therefore, you put the roll control (Ailerons) as far out on the wings as you can and the pitch (Elevators)/yaw (Rudder) control as far back/forward of the CoG as you can.

What about where to place landing gears?
You generally want to keep your CoG in between your front and rear landing gears. Make sure the gears aren't too close to your CoG or your plane might be very unstable whilst on the ground. Your rear landing gears should be placed somewhere between the CoG and your elevators (Assuming your elevators are at the rear). Too close to your elevators and it'll be difficult for you to lift your plane's nose up (because moments), too far and a tail strike might occur.


What engine should I use?
Turbojet - fast but fuel guzzling plane.
Turboprop - slow but more efficient plane.
Turbofan - fast (but not that fast) but efficient plane, also, it's supposed to be quieter.
Turboshaft - that's for helicopters.
Turbo Ramjet - SR71.
Pulsejet - well, they're cheap!
Ramjet - that's for missiles (you need air flowing into the ramjet before it can start).
Scramjet - that's for supersonic missiles.
Shcramjet - my fecking god, how fast do you want to go?
Auxillary power units - they're tiny turboshafts that generate electricity.
Electrical motor driven propeller - Why would you want an engine that doesn't guzzle fuel?
Rocket engine - If you wanna go to space (generally speaking, its a rocket engine if it consumes fuel and oxidizer).

Fun fact, the "turbo" prefix refers to the engine being driven by a turbine, which is a rotary device used to drive the engine. The turbine powers the engine's compressor section which compresses air that is then sent to the combustion chamber. The exhaust is then used to spin a turbine which starts the cycle again.

Helicopters?
I don't know how they work, they're inefficient, magic, flying bricks.
But honestly, I don't have a lot of experience with helicopters IRL, so I can't really say much about them. But the basics are, if your main rotor is rotating clockwise, make sure you've got a tail rotor or another main rotor (tandem rotors) that counters the clockwise movement of the rotor so you won't spin out of control. Also, your main rotors are what will make your air thing fly instead of wings and stuff. You'll have to try and balance your air thing so that your vertical CoT is in line with your CoG, or else it'll flip out.

What about a VTOL air thing?
Just keep your vertical CoT inline with your CoG. If you have an engine for horizontal flight and an engine for vertical flight, you can set the thrust limiter on your horizontal flight engine to zero to see the CoT of your vertical flight engine. Also, you'll have to balance your fuel tanks so that while the fuel is being consumed, the CoG won't shift much from it's original position, keeping the vertical CoT and CoG inline throughout the entire flight

What about an air-space thing?
An air-space thing is basically a space thing that can fly... So, like an air thing with rockets... Or a space thing with wings...

To summarize, you've just read about some science words, forces and how to design space and air things. Now your space and air thing designs should experience a 0.005% decrease in the probability of experiencing a catastrophic failure.

sources
My Diploma in Aerospace Engineering and design.
https://en.wikipedia.org/wiki/Bill_Nye#/media/File:Bill_Nye.jpg

FAQ

You ♥♥♥♥♥♥ up! there's a mistake here
Feck...... Looks like I gotta tear up my diploma and redo vocational school again... But yeah, I'd expect there to be ♥♥♥♥ that may be wrong because I wrote this guide whilst slightly drunk...... If you spot one, do let me know and I'll fix it. If you spot multiple, well, ♥♥♥♥... I'll fix them all and double check the stuff in this guide again...

I didn't follow your guide and my space/air thing worked! Fook you!
It's engineering, not everything is set in stone. If it works, it works.

The ♥♥♥♥ is a space/air thing?
Basically any rocket/aircraft/SSTO/whatever the fack people call them in KSP. I call them things as a blanket term for the crazy ♥♥♥♥ the KSP community builds.

Your diagrams suck
I lost my student licences for autoCAD, Creo Parametric and Ansys after getting my diploma.... So you'll have to do with some pictures from my old projects and microsoft powerpoint drawings...

You didn't really talk about how lift is generated and I wanna learn about it but I don't wanna read multiple papers on it
It's ♥♥♥♥♥♥♥♥♥ magic, I don't know how it works, no one does Okay, so for lift, it's essentially a force generated by a fluid moving across a solid object. Air has to be moving across the surface of a solid for lift to be generated (when a wing stalls, it loses lift as the air is no longer moving smoothly across the surfaces of the wing). There are various theories out there, such as "the wing's bottom surface 'pushing' the air down' and generating lift", "the air having to travel further across the top of the wing than the bottom, thereby making it move faster and creating an area of high velocity air with low pressure above the wing and an area of low velocity air with high pressure below the wing as per Bernoulli's equation [where total pressure = static pressure + dynamic pressure, and since total pressure is constant, and dynamic pressure increases with velocity, it means static pressure decreases if air moves faster], resulting in a difference of pressure that produces lift", and "the fluid is being 'turned' by the wing, thereby generating lift due to the fluid being 'turned' at the top and bottom surfaces of the wing, generating lift". These theories are called the 'skipping stone theory', the 'equal transit theory' and the 'flow turning theory'. In my opinion, the flow turning theory is the most correct out of all the simplified theories out there, but I know many subscribe to the 'equal transit theory'.

What mods did you use in your screenshots?
(this was added about a year after I made this guide)
I can't exactly remember what mods I had when I made those screenshots (I had a ton of mods), but it should've been the Quiztech Aerospace Pack for the F22 cockpit and engine, Procedural wings for the F22 wings, and BDArmoury for the weapons. As for the Untitled Space Craft cockpit, I can't remember what mod it was from, might've been the Mk2 expansion mod.

There's a seplling/grammar error in your guide
♥♥♥♥... notify me and I'll fix it

How do aircraft engines work?
Suck squeeze bang blow.

Can I add you on steam?
Depends on what your motives are.

What's the airspeed velocity of an unladen swallow?
Depends, african or european swallow? what airspeed are you referring to, Indicated / calibrated / equivalent / true airspeed? what situation is the swallow in, in flight, landed, in a wind tunnel, etc? what is the average force the swallow can output from its wings? what direction does the swallow output the force from its wings? how heavy is the swallow? whats the average drag that the swallow produces? and are we talking about a specific swallow or just on average?