Use a wire cutter to cut a piece of wire mesh about a foot longer and wider than the squirrel's entry hole. This will serve as the base mesh of the one-way door.
Mark a section in the middle of the base mesh to serve as the frame of the one-way door. The frame should be just a little bigger than the squirrel's entrance hole. Snip only three sides of the frame and fold out the cut portion along the fourth side. The folded-out portion will serve as one of the walls around the one-way door.
Cut additional sections of wire mesh to form the other three walls around the door. Attach the walls to the base mesh by folding the protruding wires or tying them down with wire strands.
Cut another section of wire mesh to serve as the swinging door. It should be longer than the door frame to allow an overlap to the wall, so that a squirrel trying to get back in can't squeeze under it.
Attach the topside of the swinging door to the base mesh; make sure the door can swing freely.
Slide the spring into the metal rod and attach the rod to the junction point of the swinging door and base mesh. Set the spring to allow the door to open when the squirrel attempts to go out, but keep the door closed so the squirrel can't get back in.
Use a staple gun to install the one-way door over the squirrel's entrance hole.
Keep the one-way door over the hole for two to three days. This will ensure that all the squirrels have exited because they have to go out to eat.
Replace the one-way door with a permanent patch by stapling down an aluminum sheet that the squirrels can't gnaw through.