Remove the stick and string just before the material dries. Smooth out the area then let the patch dry thoroughly. Apply a good grade of cement to one piece of 2 x 4, then insert it through the hole. Tie it to another piece of 2 x 4 holding it parallel in front of the wallboard. Allow the pieces of 2 x 4 to remain tied in this position until the cement dries.
Most cements require about one hour to dry.
The cement will hold the back piece of 2 x 4 firmly in position, providing a support brace for the wall patch. Now cut a patch block to the exact dimensions of the sawed-out area. The block will be slightly smaller than the hole itself, but cut it to fit as tightly as possible. Apply cement to the back of the patch block and the support brace, then put the patch into position in the hole. Work the patch compound thoroughly into all cracks.