Place green board at the ceiling with the length running horizontal to the floor.
Drill galvanized screws through the green board and into the wall frame studs. Place screws every 6 inches on each stud.
Hang a second green board panel below the first and attach it as you did the first. If the green board is too wide to fit horizontally between the top one and the tub, cut it to fit.
Trim the green board by scoring it where you need to remove extra sections. Use a utility knife to cut it.
Smooth drywall compound over the green board panel seams once they are all installed. Use a taping knife to apply the material.
Place drywall tape over the compound and flatten it down with the taping knife. Apply a second layer of compound over the tape and let it dry until the next day.
Sand the surface, then cover the seams with an additional layer of compound. Use a wider taping knife and taper the substance on the edges away from the seam. Let the surface dry.
Squeeze a bead of silicone caulk along the top edge of the tub where it meets the green board. Flatten the bead of water proof caulk down with your finger. Paint the wall when you paint the rest of the room.