Make sure you have NodeJS installed.
Create two folders, one called skins and the other called capes.
Place your skins/capes in their respective folders and name the files username.png, and replace username with whatever account you want to have that skin/cape.
Run the script by typing node index.js.
Edit your hosts file and add the following entry:
ip_address s3.amazonaws.com
Replace ip_address with the IP address of the machine that is running the script (if it running on your current machine, put 127.0.0.1).