There are as many ways to gutt a squirrel as there are hunters. I'm not saying my way is the best way, but it has worked for me over the years.
I usually start by putting on a pair of rubber gloves, especially while hunting to keep my hands clean. Start by cutting off the tail. Save it as a trophy or use it to tie flies for fishing. There's a company that will purchase your tail collection for that very purpose. Next cut a slit about an inch above the tail. I sometimes find this cut the most difficult part of the process. Unlike a rabbit, a squirrels hide is amazingly tough. I strongly recommend gutting the squirrel as soon as possible after bagging it, before regor mortis sets in. After that the fir is extremely difficult to separate from the meat.
Put the point of the knife into the slit and cut the fir up to the back of the head. Peel back the fir. I sometimes will use a pair of pliers to help grip the fir. Try not to cut into the meat or you could puncture the organs and make a mess.