Step 1: Tape the corner
Start at an inside corner. Use adhesive-backed drywall tape to cover the corner. Measure the entire length of the corner to be finished and cut the tape to that length. Fold the tape in half lengthwise, fit the crease of the tape into the corner, and press firmly so that the tape extends from the corner onto each face of drywall.
Press the creased joint tape into the corner for a tight fit.
Step 2: Apply the joint compound
Apply the joint compound from the corner out, feathering the compound out four inches from the corner on either side.
Apply the compound from the corner out on each side.
Step 3: Feather and sand the next layers
When the compound is dry, sand it with a 120-grit sandpaper attached to a wide sanding block. Wipe away the dust and apply a second coat of compound using an 8-inch compound knife. Feather the compound out even further on the second coat. Once the second coat is dry, sand again using the 120-grit sandpaper. Wipe down the wall and apply a third coat of compound, again feathering out beyond the edge to make a completely smooth finish. Once it is dry, sand the surface with a 200-grit sandpaper so that it is ready for priming and finishing.
Feather out the next two coats of joint compound with an 8-inch joint compound knife.
Step 4: Apply metal bead to an outside corner
Measure and cut a length of metal corner bead to fit the outside corner. Apply it firmly to the corner and secure it with metal drywall screws driven every five inches through the metal bead and drywall, and into the stud behind it.
Press the metal corner bead in place and secure it with drywall screws
Step 5: Cover and sand the outside corner
Use a 4-inch finish knife to spread out joint compound about four inches from the corner edge or nose of the bead. Feather the compound onto the wall as you go. Once dry, sand the first layer with 120-grit sandpaper. Wipe away the dust and apply a second coat, using an 8-inch compound knife and feathering the compound out two to three inches beyond the edge of the first coat. Sand with the 120-grit sandpaper when dry and apply a third coat for a smooth finish, using the 200-grit sandpaper for a paint-ready surface.
Sand each coat when dry for a smooth, feathered corner.