If you are patching the wall or ceiling, measure the square or rectangular area of the wall that you cut out from Step 2. Use these dimensions to cut a replacement piece of drywall to fit, 2" longer and 2" wider than the hole. Lay the replacement piece of drywall on a flat surface with the backside facing up. Measure in 1'' from all four edges and draw a line using a pencil. This should form a shape the size of the hole. Using a straight edge and utility knife, cut through the backside paper and the drywall gypsum, but not the front-facing layer of paper. Using a putty knife, peel away only the backside paper and gypsum layer. Be careful not to tear the front-facing paper.
If the patch isn't a perfect fit, place it against the hole and trim to size with a utility knife.
Holes bigger than 6", up to 12'', require a slightly different process because the patch needs more support. Using a drill, create two small holes through the piece of replacement board. Feed a piece of string through and tie both ends to the middle of a stick. Allow for about 8" of string between the board and the stick. The "stick side" of the board will be the front. The stick will be used to hold the patch in place. Twist the stick to apply pressure to the rear of the board. This will steady it in the hole. Apply a smooth coat of cement adhesive around the edges. Insert the patch into the hole and position it so the cement adhesive firmly grips the solid area around the rear of the hole. Turn the stick clockwise, twisting the string and increasing pressure against the patch board at the rear of the hole. This will hold the board firmly in place until the cement adhesive dries.
To make it easier to insert the wallboard material through the hole, be sure to hold it at an angle.
If you're working with a smaller hole (up to 6"), apply a thin layer of joint compound around the hole. Place your patch into the hole. Using a putty knife, work the paper edge down into the compound. Feather the edges of the compound and allow it to dry. Sand lightly with a fine-grit sandpaper and apply a second layer of joint compound to finish the repair.
For larger holes (between 6" and 12"), allow the cement adhesive to thoroughly dry then fill in the area with joint compound. Smooth out the area then let the patch dry thoroughly.
You may need to apply two or three layers of joint compound to build up the patched area. Always allow each layer to dry before applying another.
Let the stick and string remain where they are during the patching process. You can remove both just before the material dries.
When the area is completely dry, sand off any high spots using fine-grit sandpaper and a sanding block.
Damaged areas larger than 12 square inches may need a complete replacement panel installed because large pieces of drywall need to be anchored to wall studs or ceiling joists. It depends on where the damage is located on the wall or ceiling and how you had to cut it out. For example, if the space where you removed the damaged drywall exposes studs or joists where it will be possible to secure a patch by screwing it into the stud or joist, you can do that. If not, you may have to replace the whole drywall panel.
To replace the whole sheet, remove the damaged drywall completely, down to the studs. Remove the drywall screws that anchored the old sheet to the studs using a power drill. If there was a lot of water damage, this should be fairly easy; the old drywall will most likely crumble away.
Measure the height and width of the space you’re repairing so that you know how much new wallboard you need. If you need custom-sized panels for an irregularly shaped repair, use your measurements to cut a patch to fit, using a utility knife or drywall saw. Align your new panel with the wall studs and then secure the panel to the studs with drywall screws and a power driver. Use joint tape to cover the seams where each panel meets. Then apply joint compound over the tape, using a putty knife. Let the compound dry for the time specified by the manufacturer and then sand the compound with fine-grit sandpaper until the seams are flush with the drywall panels.