Custom Skins Loader Mod
This mod lets you simply place your skins and capes in a folder to edit the skins in the game. The skins can be reloaded instantly at any time by pressing F5 in-game.
To see all of the skins and capes you can replace, press Shift+F5 in-game. This will create a skins folder in your ~mods folder with all of the default skins.