Me and my dad made a sling out of some kite string and the leather patch off the back of some old jeans when I was young. Pretty simple to do. Just attach a couple of 2-4 foot pieces of string to both sides of the leather patch or anything else you can use to hold the ammo. Put a small loop on the end of one fo the strings to loop around your wrist or thumb and you are ready to go. Takes more practice to get the aiming down, but after a few hours of fiddling with it, My and dad were able to knock Campbell cans over from about 20 yards. Left a sizable dent in them too. Definitely take out a squirrel, but you'd need to be farther away I expect. Not too many squirrels are going to stick around once you start swinging that thing around.