No-Return
You will need some wire called hardware cloth, 24" X 16". Hardware cloth comes with either ¼" or ½" holes, you can use whichever you can purchase. Using a square 4" X 4" X 24" long, wrap the wire around the 4 X 4, being careful to product square corners. Remove the 4 X 4 and secure the open corner, producing a square tube 24" long. Now cut the corners on one end so that you have four 4" flaps. Fold the flaps out so that you can use them to secure the tube to the fascia board or roof. Next make a door that fits inside the tube on an angle of about 45º, and fasten it in place so the squirrel coming from the flap end can push the door up and leave. The door must work easily and not stick on the sides of the square, but must be a close fit so the squirrel can't squeeze through the sides. The door should be installed so that the squirrel can come out and stand in the end of the square and the close behind him. If you have done the job properly, the squirrel will not be able to get back into the house. Naturally, you will need to locate all of the entrance holes and either seal them or install a no-return.
Locating the entrance hole of squirrels can sometimes be a big problem, but this is the first step to getting rid of them. Check the entire perimeter of the roof along the top edge of the fascia board they sometimes chew an opening here. Next check each side of any dormers where they can get up under the shingles and flashing. Now check all the vents they should have screening on them. It's easier to check the vents from inside. You can usually tell which hole they use most because the wood is smooth and darker around the opening. All but two holes should be plugged with heavy wiring like the stainless steel pads women use to clean pots. Pound it in real tight or they will pull it out and re-enter.