Step 1
Embed paper drywall tape over all joints between boards of drywall. For example, use a 6-inch drywall knife to apply a thick layer of joint compound to the interior angles at the top corners of the window surround. Fold pieces of paper drywall tape lengthwise and push them into the corners. Wipe the pieces of tape flat with the drywall knife.
Step 2
Apply a thick coat of joint compound over both sides of the corner bead, using the 6-inch drywall knife. Fill the sides completely. This initial coat will be rough, which isn’t a problem because subsequent coats will cover it entirely. Allow the first coat to dry.
Step 3
Apply another thick coat, using a 12-inch drywall knife to cover a larger area. Leave the coated corner bead smooth and even. If your work is rough, add more compound and try again. The smoother this coat is, the easier the next coat will be. Allow the second coat to dry.
Step 4
Sand the dried compound with 100-grit sandpaper until it is smooth.
Step 5
Water down your joint compound until it has the consistency of pancake batter. Apply a coat of thinned compound, using the 12-inch knife. Immediately wipe off the compound, leaving the corner bead with a smooth polish. Allow the third coat to dry.
Step 6
Sand the dried compound with 150-grit sandpaper. Use a folded piece of sandpaper to rub the ridge of the corner smooth and to remove any thick edges. When you finish, the window surround should merge seamlessly with the surrounding wall.