Larger holes in wallboard require some type of supporting brace for the patch.
Use a short piece of 2 x 4 cut to the proper length as a supporting brace for patching a large hole in plasterboard ( Fig. 1 ).
Cut two pieces of 2 x 4 to a length about 8" longer than the distance across the hole.
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.
Next, remove the supporting piece of 2 x 4 in front of the wallboard by untying the string ( Fig. 2 ). 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. ( Fig. 3 ). 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.
Use a firm putty knife or patching spatula to apply joint compound all around the patch board

( Fig. 4 ).
Work the patch compound thoroughly into all cracks. Scrape away any surplus material, then allow the patched area to dry completely.
When the area has completely dried, use a regular sanding block and a piece of fine sandpaper to sand away any high areas on the patched surface ( Fig. 5 ).
A prime coat can now be applied to prepare the wall for painting.