First of all, I can't stress this enough, the game is short enough for you to discover it by yourself.
You'll have it finished in a couple hours by taking your time, and maybe an additional playthrough after you discover how to be worthy. The game is voluntarily not telling you in clear way what to do, where to go and that's for a reason, it's quite rewarding to actually figure everything out by yourself up until the very end.
Come back to this guide if:

• something you're having trouble to understand;
  
• you get lost;
  
• you want to get most achievements.

This guide will basically spoil all the content there is in this game so be warned.

There are 2 savefile slots you can load or erase anytime.
There are basically a couple ways you can save the game:

• hit the bell in the dedicated bell rooms;
  
• change zones (going through a door will act the as hitting a bell and save the game).

You have 3 health points, a single health point lets you get hit 2 times.
When you die you go back to the place you last saved, see above.
You can regenerate health points in several ways:

• sometimes enemies loot an orb, hit that orb 3 times to regenerate one health point;
  
• go hit a bell to save game, restores full health;
  
• collect a health pick-up, restores full health;
  
• kill a boss, restores full health.

There are 3 additional health points you can obtain, refer to the map in next section to know where to find them.

There are 8.5 areas:

• Outdoor/Hub, where you start
  
• The Sanctum
  
• Red Jungle
  
• Emerald Mines
  
• Dead Jungle
  
• Ancient Oak
  
• The "Library"
  
• Are you worthy of SKIN?

You can find maps for each of the 5 areas written in bold. Pressing ENTER will display the map for the current area.

• Move: Use arrow keys (available at start)
  
• Attack: Press either X or UP+X (available at start)
  
• Jump: Press Z (available at start)
  
• Dive: While airborne, press Z or C;
  Let you get speed mid-air to reach platforms you wouldn't with a simple jump.
  
• Backflip: While grounded, press C;
  A second type of jump which makes you go higher, can not be used in water.
  
• Dash: While walking, press SHIFT;
  Let you run to destroy fragile rock blocks.
  
• Ground pound: While airborne, hold DOWN and press X;
  Downward attack from mid-air, to destroy fragile rock blocks and kill enemies.
  
• Bowl: Hold DOWN and press Z or C;
  Launch your head bowling style, to go in narrow spaces, you can only exit bowling form by knocking on a wall.
  
• Blast: While grounded, hold DOWN and press X.
  Repels you upward until you reach to roof, to rapidly gain height.

If after wandering for hours you still don't understand what you are meant to do and how to finish the game.
Not spoiling the last cutscene, you have to use the last skill "Blast" you get, outdoor.
To get this skill you first have to hit all the 5 gongs present in each area of the game to unlock the second and last boss in the "Dead Jungle" zone.
Once the boss is beaten, reach for the Blast skill and go back to the starting hub in order to use it outdoor.

Though depending on the way you complete those task, there's something else you can unlock, more details in the achievements section.

Here's the Gimp .xcf file[filebin.net] in case you want to build upon it, lots of layers.

Get the "Dive" skill pickup in the Red Jungle.



Get the "Backflip" skill pickup in the Emerald Mines.



Get the "Dash" skill pickup in the Red Jungle.



Get the "Bowl" skill pickup in the Sanctum.



Get the "Ground Pound" skill pickup in the Emerald Mines.



Get the "Blast" skill pickup in the Dead Jungle after the last boss.

Collect all Life Orbs

Find the 3 Life Orbs in Red Jungle, Emerald Mines and Dead Jungle (check map section).

Collect all Map Orbs

Find all 5 maps pick-ups in each named area (check map section).

BEFORE HITTING ANY GONG.
First you could head to the door not link to other area in the Red Jungle, you need "Dash" to break rocks.

Then you should head to the "Library" (self-named by myself) the question mark on the map of the Sanctum. There you will find 5 pictures which are hints about how to get WORTHY OF SKIN.

The 5 hints point to each gong, however each gong is assigned a letter.
With those 5 letters you can spell the word H-U-M-A-N, which gives you the order in which to hit the gong.
After you hit all 5 gongs in the correct order, just go back to the area I mentioned in the Red Jungle, you should arrive in a different area than before which has the Skin pickup at the end.
I don't think there's a way to actually keep the Skin, in Steam discussions the developer mentioned it was a red herring from previous games. :D

Complete the game within 17 minutes
I don't have any advice for how to achieve the speedrun.
Though I recommend you check the speedrun leaderboards[www.speedrun.com], there are videos for each entry and it even has some guides for various techniques or advice[www.speedrun.com].