If the skin is loose I make a cut right in the middle of the back. Put two fingers in and pull it apart. Pull skin down over back legs and cut off at feet. Pull down over front legs and cut off at feet and head. Pull the rear part out over the tail if you're saving the tail. Then gut and rinse and you're ready to cook.
If it's tight I make a cut at the base of the tail, through the bone. Be careful not to cut through the skin. Then hold the squirrel by it's hind legs, step on the tail, and pull by hind legs. Usually this results in the whole squirrel pulling out of the skin as long as it doesn't rip.
-Jake