Capes

Set Cape

File must be a PNG, using cape dimentions, see Minecraft Wiki

Set Cape (via existing)

Get Cape

See Everyone's cape

Example Cape

For if you wanna make your own cape.
The dimentions just have to stay in the same aspect ratio.
You have to reload the current world to update your cape
(changing lobbies, going from overworld to nether, etc)

Blue: Front
Red: Back
Purple: Sides
Aqua: Bottom
The Elytra: Well, the elytra, not used because we are on 1.8.9