Examine and measure the space you want to fill in the room (Image 1). If the project involves complicated cuts, consider creating a diagram to make it easier to transfer the measurements directly to the drywall (Image 2).
After measuring carefully, score the surface with a utility knife and sharp blade, making sure to cut through the top layer of paper (Image 1). Once the board is scored, simply bend and snap it. That will leave it hanging by the bottom layer of paper. Use a sharp blade to make a clean cut through the paper layer (Image 2). For more complicated cuts, like holes for switches or vents, use a saber saw. After measuring and marking the board, carefully sink the blade directly into the board (Image 3). If you don't have a saber saw, you could also use a small, sharp-pointed hand saw.
Attach the board to the wall-studs using cupped-head nails. Set the nails by sinking them just below the surface of the wall, making a slight dimple with the head of the hammer. To get the smoothest possible finish, fill these slight depressions with joint compound, commonly called drywall mud. The compound comes pre-mixed in various sized tubs. Use a taping knife with a wide, flexible blade to sweep the compound over the nail hole. This process covers the nail head, which should be just below the surface of the wallboard.
Fill the space between boards with joint compound, making sure to spread a thin layer about two inches wide on either side of the seam (Image 1). Next, place a strip of drywall tape from top to bottom, centered over the seam (Image 2). Use a stiff blade to set the tape firmly into the mud and get it as smooth as possible; then use a wider blade to sweep a thin surface of mud across the surface (Image 3).
Apply drywall compound to the corner seam using the same technique as in the previous step (Image 1). In this demonstration, we used a special 90-degree drywall tool, available at home improvement stores, to fill the corner seam with a layer of mud (Image 2). The tool is designed to smooth the drywall compound over the tape and down the wall with one motion. Fold the tape into the corner (Image 3) and smooth it over. Allow the compound dry for a day before applying primer and paint.