Inside corners are among the trickiest parts of a drywall project to get right. As a result, they are often places where you encounter problems later. If you get cracking seams in the corner, buckling tape or other problems, the only solution is to remove the tape and plaster and completely and redo the corner. The process of reapplying the tape and joint compound (a form of plaster) is the same as the process used in the initial taping of the drywall.
Run the edge of your drywall knife down the centre of the corner, breaking out the plaster and exposing the tape. Get the knife under the tape from one side of the corner or the other. Use your razor knife to get under the paper if necessary. Pull the paper off, and scrape off any remaining plaster, completely exposing the corner seam.
With your 6-inch knife, spread two lines of joint compound lengthwise on both sides of the corner, with each of the lines being about 3 inches wide and 1/8-inch thick.
Pull out an arm's-length piece of paper drywall tape. Crease it in half lengthwise. Press it into the corner (in the bed of compound). Repeat the process with other lengths of paper tape until the whole corner is covered.
Press your corner taping knife into the corner at the top, on top of the paper, and drag the knife down slowly, holding it at a slight angle away from the wall. Flatten out the paper while squeezing excess joint compound out from under it. Do the whole corner.
Spread a second coat of compound over the top of the tape in the same manner as you laid the first. Run the corner knife over the compound to form it into a cornered shaped. Let it dry to the touch.
Sand the compound smooth with your drywall sander, running it along both sides of the corner in short, light strokes. Re-apply a second coat of compound, making it about an inch wider than the first. Run your corner knife over it. Let it dry.
Sand as before, apply a slightly wider third coat of compound and run the corner knife over it. Give a light final sanding after it dries.
Wear a dust mask during the sanding.