Click on someones else's mob skin and then on the side near the top will be a blue button labelled create a ------- mob skin, the --- sign is like a blank because it depends on which mob skin you click on. When you click the create button it will ask you which mob you want to do.

hope that helps,
-AB2

where is that folder?

Go to (Your resourcepack)\assets\minecraft\textures\entity, pick the entity you want to modify and edit it's png file. Good luck :)

HELP