IF you play offline or have a cracked minecraft go into your .minecraft folder, click bin. Then access the minecraft.jar with 7zip or Winrar. INside you will see a folder called Mobs, click it and open it up there is an image called char, rename your replacement skin this and make sure its int he same format ad the one in the minecraft.jar all you now do is drag drop and replace the old vanilla minecraft char image with your new skin.
REMEMBER back up your minecraft before doing this just encase something goes wrong