Fixing a drywall crack at the wall/ceiling joint will improve the looks as well as the structural soundness of your room. Drywall is a construction panel made from gypsum plaster and thick sheets of paper that is then dried in a kiln. Drywall is easier to install than traditional plaster, and is easier to repair or replace as needed. While drywall will not crack like plaster will, you may still see cracks wherever the drywall sheets join, such as at corners and at the ceiling line.
Remove or cover all furniture. Take down curtains and window treatments. Sanding drywall is a messy job, so the more you prepare, the less cleaning you will need to do later.
Use a utility knife to scrape all of the lose pieces of drywall out of the crack. For best results widen the crack so that it is approximately 1/8 of an inch.
Put the joint compound into the crack in the drywall joint with a small putty knife. Depending on the depth of the crack you may need more than one coat. Let each layer completely dry before adding another (one to two hours). Do not let the patch become raised over the crack; it should be flush with the wall/ceiling.
Wait 24 hours before sanding. Using a 150-grit sandpaper, smooth over the patch. Use light, smooth strokes so that you do not sand too much patch out of the fissure. For an extra smooth finish, go over the area with a damp sponge.