How do i make capes appear on a player? Like the minecon / mojang capes but with my own custom texture.

public static void addCapes() {
    	String capeURL = "URL_TO_CAPE.com/cape.png";
    	String[] owners = {"hotrods20", "OtherUsernames"};
    	
    	ThreadDownloadImageData image = new ThreadDownloadImageData(capeURL, null, null);
    	
    	for(String username : owners)
    	{
    		Minecraft.getMinecraft().renderEngine.loadTexture(new ResourceLocation("cloaks/" + username), (TextureObject)image);
    	}
    }

 

Put that in your client side proxy.  Don't forget to put this.addCapes(); to your load method.

Sources of info:

http://www.minecraftforum.net/topic/2254009-164-forge-darkhaxs-modding-tutorials-how-to-add-capes/

EDIT: Learn to Google a bit before asking.