To get started with SlipCover you will need to amass a collection of high resolution images that you will be using as the source for the icon. Movie posters make a good choice if you are adding icons to movie files. Album covers for music folders etc.
Slipcover comes with 3 default icon templates but you can download more check out the selection at http://dustinschau.com/#rw01. If you are looking for an easy, free way to add custom icons on your Mac, SlipCover does the job perfectly.