Scrap bits of crumbling drywall out of the crack and pull off wallpaper, if applicable.
Cut a piece of fiberglass drywall tape to fit over the crack. This mesh tape is self-adhesive. If the crack branches, apply additional strips of tape, but do not overlap the ends. It’s okay if you leave small gaps of 1 inch, or less, uncovered.
Apply premixed joint compound with a 6-inch taping knife directly over the fiberglass tape. Press it through the holes in the mesh and into the crack beneath. Use as much compound as needed to fill the crack thickly and evenly.
Wipe off excess joint compound from the surface of the wall with the wide edge of the taping knife.
Change to the 10-inch taping knife after you fill the crack with compound. This wider knife spreads the compound on the surface of the wall over a larger area, which reduces lumps.
Sand the crack after the joint compound is completely dry with a drywall sander. This type of sander features a pad, covered by a replaceable screen that quickly removes dry compound bits and evens out rough spots.
Apply another thin coat of joint compound with the 10-inch taping knife. Spread this coat quite thin, using just enough compound to lightly cover the first coat. If you can see through it, that’s not a problem.
Sand the crack again after the second coat dries and add one or two more thin coats of joint compound, if you can see or feel imperfections over the crack. The compound contracts slightly as it dries, so multiple thin coats will give you better results than one thick coat.
Repaint or touch-up the paint on your wall.