Measure the length of one of the garage walls. Cut two 2-by-4s to this length to use as a top and bottom plate.
Place the 2-by-4s on the floor on their edges and align them together. Measure along their length with a measuring tape and make a mark with a pencil every 16 inches.
Measure the height of the garage wall. Cut the studs to this length, minus 3 inches.
Move the top and bottom 2-by-4 plates apart and put the studs between them with the top and bottom of each stud aligned with the pencil marks.
Screw the studs to the top and bottom plates using two 3-inch-long screws driving through the faces of the plates and into the ends of the studs.
Stand the frame up and put it in place against the outside wall. Secure it by screwing down through the bottom plate into the floor and up through the top plate into the ceiling joists.
Insulate between the studs with batts of fiberglass or mineral wool.