Gently peel back the drywall paper, and hold it in place with a piece of tape or your hand. Don't tear it off the rest of the drywall. If the paper is missing entirely go to Step 7.
Take a small amount of mud, and apply it to where the drywall paper has become loose from the rest of the drywall.
Gently press the drywall paper back in place. Some of the mud will most likely ooze out of the sides. Wipe this away with a drywall knife.
Allow the mud to dry.
Sand the paper down if needed along the edges so that your wall is level and smooth again.
Apply another thin layer of mud around the torn edges, and then repeat this process if needed.
If you're missing the drywall paper entirely, simply apply a thin layer of mud to where the paper is missing. Let this dry, and then apply more mud if needed.
Once the wall is smooth and level again, seal the drywall and mud with a good primer. This will prevent the drywall paper from tearing in the future.