Demolition
Step 1
Remove the baseboard around the base of the opening. A thin glazing bar similar to a flat bar, but straighter and not so thick, works best because it can be slipped behind the baseboard without damaging the wall.
Step 2
Remove the corner bead by cutting along the edge of the corner bead with your utility knife and removing any screws or nails that hold it on.
Step 3
Remove the drywall inside the opening with a flat bar. The jamb is attached there and should not be installed against drywall.
Jamb Installation
Step 1
Nail your jamb together with 6d finish nails. Jamb material comes disassembled and is put together on site. Cut the overall width of the jamb about 3/4 of an inch narrower than the rough opening to allow for shims. If the side jambs are cut out to receive the head jamb, add twice the depth to the length of the head piece.
Step 2
Place a pair of shims together so that their thickness is 3/8 of an inch, and nail them 1 inch from the bottom of one of the studs with 4d finish nails.
Step 3
Place your level against the bottom shim while placing two more shims at the top of your level, adjusting the shims until the level is plumb (the bubble in the center of the glass). Nail the shims with 4d finish nails.
Step 4
While holding your level against the shims already nailed to the wall, slide in two shims up tight to the level and nail them to the wall. Place shims every 18 to 24 inches and 2 inches from the top.
Step 5
Set the jamb in the opening and check the head piece for level. If it is level, drill pilot holes a little smaller than the nails, and nail the jamb through the shims into the wall with two 8d finish nails spaced 2 inches apart. If it is not level, cut the long leg shorter with a circular or hand saw.
Step 6
Place shims at the top of the other side of the jamb, and nail though the jamb and shims with 8d finish nails.
Step 7
Place your level against the jamb and install shims at the bottom of your level. Adjust the shims until the jamb is plumb and nail it to the stud with 8d finish nails. Repeat this process until the shims are about 18 to 20 inches apart. If your opening is wider than 3 feet, shim the head jamb also.
Installing Casing
Step 1
Cut a piece of casing with a 45-degree angle on the end, and nail it to the jamb with 4d finish nails and 8d finish nails into the wall stud. Nail along the jamb every 8 to 10 inches, and into the wall 16 inches apart. To determine the total length of the casing, hold a scrap piece of casing on the head jamb so that there is a 3/16-of-an-inch reveal, and measure to the top of the casing.
Step 2
Cut a 45-degree angle on both sides of the head casing; attach it to the jamb with 4d finish nails and to the wall with 8d finish nails.
Step 3
Cut the baseboard to fit tight against the casing, and nail it to the wall with 6d finish nails.