Squirrels are good because they are cheap, easy to get and relatively tough. On the other hand, they are small, making it more difficult to see what you are doing. This is a good time of the year to get your hands on some deer heads that are going to get tossed out. Practice skinning, turning and fleshing on them. It will be easier to see what you are doing and no loss when you mess up.
Lots of good videos out there too.