This mod will not only let you get a cape but let you see other peoples capes and anyone who has this mod will see your cape it only works on minecraft PC
Step 1: Forge
To make this easy you should download and install the latest version of forge (if you haven't already) you can get it at files.minecraftforge.net. I suggest getting the installer for the version it is. Then start minecraft you may have to reenter your username and password and make sure the profile is "forge"