Scrape off the plaster from the area of the crack with the putty knife. Scrape off enough to completely expose the paper or mesh tape underneath and pull it off. You should be left with an exposed seam between the two pieces of drywall.
Press mesh self-stick drywall tape along the seam, covering it completely.
Spread a layer of joint compound along the tape with your 8-inch drywall knife, covering it completely and getting it flat and smooth. The line of joint compound should be just slightly wider than the tape, about 4 inches. Allow the compound to dry for eight hours.
Scrape off any ridges or high spots from the dried compound with the drywall knife. Apply a second layer of joint compound, making the seam slightly wider than before while keeping it thin and flat. Let the second coat dry for eight hours.
Scrape off any ridges with the drywall knife. Apply a third coat, wider and flatter than before. Let it dry eight hours.
Sand the joint compound by hand with your drywall sander. The ceiling is now ready for repainting.