Embed paper tape in joint compound using a 6-in. taping blade. To avoid trapping air bubbles under the tape, moisten the paper tape with water, lay it over the crack and squeeze excess compound and air from underneath with the blade. Apply an additional thin layer of compound and feather it off 2 in. on both sides of the tape. Let dry.
Apply a second (and third, if necessary) coat of compound, smoothing it out 6 to 7 in. on both sides of the joint. Smooth the compound to a thin, even coat using long, continuous strokes with a 12-in. taping blade. Allow the repair to dry thoroughly, sand it smooth (avoid exposing the tape) and paint it.
As homes settle, cracks may radiate from the corners of doors and windows. Whether your walls are made of plaster or drywall, you can repair the cracks in two steps over a day or two—and get the area ready to sand and paint. Use paper tape; it's stronger than fiberglass tape for wall repairs. For cracks more than 1/4 in. deep, clean out the loose material and use a quick-setting crack filler like Durabond to build up the area level with the wall. Then use the steps shown in Photos 2 and 3 to fix it.