Framing for a Window - Removing An Existing Window
If you've decided to take out an old wood window, start by prying off the stop around the sash. Remove the window sashes next to avoid breaking them when taking off the rest of the framing.
NOTE: An old window may have counterweights behind the side jambs. You'll want to remove them through the access panel on the side jamb.
Using a pry bar, remove the casing (trim) from the jamb frame. Remove the nails securing the jambs to the framing by cutting them with a reciprocating saw or mini hacksaw and prying the jambs loose.
Now the window can be pulled out as a unit, working from the outside. In some cases, older windows may have to be disassembled piece by piece, but most windows should pop right out.
If the window you're taking out has aluminum or vinyl cladding, it will likely be held in place by a flange nailed to the sheathing under the siding. You'll need to remove any trim and enough siding to pry the flange loose and then remove the window from the outside.
Framing A Rough Opening
The framing around the rough opening supports and transfers the structural weight above the window.
The rough opening is made slightly larger than the window's actual size and rough opening dimensions are usually provided by the window manufacture.
NOTE: Manufacturers typically recommend the opening be 1/2" larger than the window. However, for better insulating between the window and framing, 3/4" is also suggested.
If you're framing a new opening in a finished wall, you'll need to remove existing drywall/plaster down to the floor. Remove the baseboard. Use a cold chisel, utility knife or saw to cut existing drywall/plaster from ceiling to floor.
Cut the exposed studs where the bottom of the sill framing piece and the top of the header will be. These cut studs can serve as cripple studs under the sill and over the header.
TIP: For a clean cut, use a circular saw cutting as deep as possible, then finish the cut with a reciprocating saw.
Pry out the cut section and pull the remaining nails inward. Don't punch the nails out through the exterior. Next, cut the trimmer studs to accommodate the header and nail them to the existing wall studs.
Square the header in place on the trimmers and toenail it to the wall studs. Next, install the sill piece. It lies across the lower cripple studs and supports the window's weight.
Remove the section of exterior wall where the window will be and nail the exposed sheathing to the new framing.