Clear all crumbled wallboard from the area you plan to repair by cutting around it with a wallboard knife and pulling it out by hand.
Spread wallboard tape over the damaged area if it is small or thin. For large holes, form a support by sticking a thin wood strip through the hole and, while holding the strip in place with one hand, drive a screw through the drywall and into the wood on each side of the hole; use additional wood for larger holes. Cut a piece of wallboard to fit the hole, attach it to the wood and tape along the seams.
Use a drywall knife to apply a dollop of joint compound to the damaged area and spread it into a thin, even coat. Allow the compound to dry before applying a second coat. Allow the second coat to dry before applying the third and final coat.
Smooth the area with sandpaper after the final coat has dried.