The best way to remove squirrels from an attic is to have the job done by a professional exterminator. To remove squirrels without professional help, use either a repeating live cage trap, which catches all of the squirrels coming in to the attic, or a one-way extrusion door, which denies entrance.
Calling an exterminator is best for squirrel removal because they are licensed to catch and relocate animals, which is not legal in all areas. If it is legal, live trapping is humane and less complicated than using lethal body grip traps.
Ultimately, getting the squirrels out and not allowing them back in is the best DIY solution. A repeating live cage trap set in front of the squirrel entry point catches each pest coming in and must be placed while the animals are out during the day. The one-way extrusion door allows the squirrels to push through to get out of the attic and blocks them from coming back in.
Because trees make it easy for squirrels to come inside the attic or get in your roof, exclude the squirrels by circling the trees near your home with a collar of smooth metal 6 feet from the ground. The squirrels can't dig in their claws into the metal, which prevents them from climbing.
One more thing to consider is whether there are babies in the attic nest. Squirrels that are blocked out may chew a new opening to get back in to their babies, which is another good reason to call a professional exterminator.