Havahart has the best page of trapping tips I ran across. If you're are trying to trap ranging squirrels you'll also want to look at Tomahawks double-door trapping tips . Neat ideas for making yard scoops. Once you remove the squirrels you have to close their entry points. You may not want to remove them before locating their entries so you can watch their activity to locate their holes.
Entry points are often in trim and at the bottom corners of roofs near the ends of soffits.
Examine trim and soffits all around. If they find a small opening in a piece of trim they can gnaw it into an entry hole in minutes. Check aluminum roof ventilators. They can quickly chew through aluminum screening. Squirrels entered the house on the right through the soffit at roof level, then into the attic, and began removing insulation from the walls.