As we may Know from speed-run's/seeing someone preforming it right between your eye's. We can see someone FLY in different direction, some flying fast, some high or sometimes both!
As we may see, (let's say during a speed-run) someone jumping on a different player, and then getting boosted all the way to the heaven's! Or somewhere very fast, while reaching low. you need to know where to : aim, look, where to position your-self or know the spot's
This one is a little more tricky, you need to know where to: aim, look, where to position your-self or know/show where too boost the player.

It's really easy unless you duck up miss the boost.
1)Where to look
Make sure too look toward's the booster. (the best is to keep looking at players face) Or forward so you don't lean to the left or right.
2)Where to Walk/Go
Make sure you have a clear path. (No common infected/special's around you)
Walk toward's your booster, remember to hold shift for easier boost!
2.5)walking without shift
It's also possible to boost without need of shift, you may also gain extra speed from it. (It's harder for booster to boost, unless he is a expert or luck may struck up.) (no gif at the moment)
3)When to jump
Some player's will Jump every time differently. (early or late) So in order to make it come comfortable, try to jump at the same time you did before. "Remember to hold space so you don't get stuck hanging"
4)Where to land
Just try to land as high as possible so you don't take too much damage, or hang.
5)Movement
In order for getting to different spot's, not going 'Over/Below' the spot you want to land in.
Make to sure 'Depending' if you want to go Further/Nearer, you need to hold either: W or S. Or none of the above keys.
Holding W: If you want to go Further/More far make sure you need to hold W, (for going even further look at number 6). Its mostly important if you want to go faster.
Holding S:
If you want to go Slower/Less far, make sure you need to hold S, (not recommended for throwables).
Not holding ANY KEYS:
This is is almost like going with "W", just a bit slower.
Here is a comparison of the boost's.
Oxygen tank is with holding W. (GREEN)
Gas Can is with holding NO KEYS. (RED)
Fire cracker's is with holding S. (YELLOW)
(all boosts where made with throwables " -36.31° ")
6)Surfing
Surfing is a method of gaining speed while in mid air by walking from left to right and also aiming left to right.
For more chek out this tutorial, (more complicated).
https://gtm.steamproxy.vip/sharedfiles/filedetails/?l=german&id=184184420
Also chek out this tutorial, [more simple].(0:07-1:35)

Before i get in to the steps of preforming boost's, we need to talk about with what we can boost.
Depending on where you want to boost and how high or fast do you want to achieve.

1)Where to look
Depending on what surface or where do you want to boost someone, up hill or down hill, (only slight downhill because the lower down hill there might be no chance of boosting). You need to look at their feet or under it.
If you are trying to boost someone up, (uphill) don't constantly look at their feet, look at the sky (there are different angels at nb.2).
If you are trying to boost someone up, (downhill) make sure to look at their feet, when they jump try to shoot. It might take a litle amount of tryes but surley it may work.
This is a example of a grenade launcher throwable boost, (i forgot to add a clip of grenade launcher)(where to look).
2)When to shot
Its more easy to time it when a player is on shift and not running.
As soon some one jumps on you shoot, make sure to there is no hitbox delay. (Check other stuff for hitbox video). Or if you don't know duck nothing about hitboxes, just try to time it when someone just jumps at you.
Make sure to look at the sky, (if you want to send the person high look very high, but if you want to send the person lower but with speed look more down).
3)Spot's
There are some spots that you may skip with your buddy/random.
You can also discover : boost's, skips or godspots (i will not show you the skips or godspots).
4)Showing someone where too boost
This is mostly done by shooting your pistol and then saying : "there" or "go there and then come to me".
Or just simply saying : "go boost here" or "go boost from here". If they know what's up.

Also you can boost infected! (Tank's and charger's can't die from a single grenade launcher shot)
but with spitter's boost and Granades you can boost all infected! (molotove's can kill inf if you fail)

This is post from speedrun wiki showing what angle if you look, you will land.
https://l4dspeedrun.fandom.com/wiki/Grenade_Launcher_Boosting
also Thanks HSTop for good concent
https://www.youtube.com/@HSTop
Hitboxes video

If you know what to add or what to fix go ahead and point it out.