Please do not report me or ban me for doing this, and I have bought the game, but I'm only making this blog to help people who haven't bought the game and wanted a skin. :)

Step 1. You need to download WinRar archive. Download it here.
Step 2. Go to "Start", then type in "%appdata%", without quotes. If you have Windows XP, click "Run" instead.
Step 3. It should bring up a folder called roaming, click on ".minecraft", and go to bin.
Step 4. Inside of bin, there should be a file called "minecraft", right click it and click "Open With >> WinRar Archiver"
Step 5. There should be a lot of different files, go ahead and click the one that says "mob."
Step 6. Click on "char.png" and open it with Paint.net, Gimp, or any other editing picture program.
Step 7. Edit the skin to your way, and Save it. When you go back to the archive, it says:"File "char.png" has been modified. Do you wish to update it in the archive?" CLICK YES.
Step 8. Close WinRar, and open up Minecraft and your edited character should be modified!
Hoped this helps~
SuperBuza ;)