Spread drywall compound on the crack with a putty knife, making several passes back and forth over the crack to ensure you work the compound into the crack.
Place a length of drywall tape in the drywall compound directly over the crack if the crack is large enough that you want to add extra reinforcement. Cover the tape with another layer of drywall compound and wipe off the excess. Allow the drywall compound to dry for the time indicated on the pail.
Sand the drywall smooth with 150-grit sandpaper after the compound is completely dry. In a successful job, you should not be able to feel any transition between your patchwork and the rest of the drywall.
Paint the patch and the area immediately around the patch with paint that matches the paint used on the wall.