Use a sharp utility knife to cut the desired access hole out of the wall. Size the access panel so that its two vertical sides are flush with existing studs.
Measure the distance between the studs. Cut two pieces of two-by-four to this length.
Place one of the two-by-four pieces horizontally between the two studs so the top face of the two-by-four is flush with the bottom edge of the access hole. Secure the two-by-four by driving 3-inch-long screws through the faces of the vertical studs and into the ends of the horizontal two-by-four. Secure the edge of the drywall by driving several 1 1/4-inch-long screws through the drywall and into the horizontal two-by-four.
Place the second piece of two-by-four between the two vertical studs so that its bottom face is flush with the top edge of the access hole. Secure it to the studs in the same manner as the previous piece of two-by-four. Secure the edge of the drywall to the two-by-four.