Climb your ladder to the damaged ceiling seam. Scrape your drywall knife along the seam, getting it under the tape and plaster. Pull off the tape with your fingers when possible, taking the plaster with it. Pull down all the tape and as much plaster as will come off, exposing the entire seam.
Press a strip of mesh adhesive drywall tape over the seam, covering it completely.
Spread joint compound over the tape, from end to end, using the drywall knife. The line of compound should be wide enough to entirely cover the tape. Make the compound flat and smooth with the drywall knife.
Let the joint compound set for four to six hours. Scrape off any ridges or high spots with your drywall knife.
Apply a second layer of joint compound over the first. Make this line of compound a little wider than the first.
Let the second coat dry four to six hours. Run over it with the drywall knife to scrape off any high spots.
Apply a third layer of joint compound, wider than the second. Get it very smooth and flat.
Let the third layer of compound set for 12 hours. Sand it smooth with a sanding pad. Paint the whole ceiling.