Back in my college days my roommates and I would shoot squirrels out of our back yard for food with a crossman air rifle. It topped out at 650 fps with pellets. Rarely would we have one that needed a second shot. We kept our range to about 15 yds. Aim right between the eye and the ear. Just like with a rifle, shoot the type of pellet that works best in your gun. Mine liked the flat, target pellets the best.
Practice is the key. we had a 15 yd "range" set up from our kitchen table down the hall into the living room (cardboard box with a thick phone book behind it). We shot everyday, making bets on shots. We started out trying to hit an outline of a quarter that we drew, then a nickle then a penny, and dime. After we could hit a dime 4/5 shots we started taping playing cards to the box. Once you can take out the diamonds on a five with five shots... you are ready for squirrels. Up for a new challenge? Try taking out the tiny diamonds on a card in the two corners just below the number "5" or "8" whatever. that's tough.