Now that you have chosen a new window for that wall you are building, you need to build a frame for it. Framing a window opening in a wall takes nothing more than the basic carpentry skills of measuring, cutting and nailing. It's not as difficult as you may think and, chances are, you'll find that you can build a window frame in less than an hour.
Look through the manufacturer's literature included with the window you have purchased and locate the rough opening dimensions that will be required. Determine the location in the wall where you want the window and then mark the space for the window opening on the top and bottom plates.
Make 2 additional marks out from the rough opening marks you just made; one at 1 1/2 inches for the jack studs, and one at 3 inches for the king studs. Cut the king studs from the 2-by-4 inch boards to the same length as your interior wall studs and nail into place.
Measure the distance from the wall's bottom plate to the planned bottom of the rough window opening. Subtract 1 1/2 inches from this measurement and cut the jack studs to this length. Nail them into place inside the king studs and below the window opening, as shown in the diagram.
Measure the distance between the king studs in the rough window opening and cut a piece of 2-by-4 inch board for the base plate to this length. Nail it into place between the king studs and on top of the jack studs. It should be flat so that the window can rest upon the wide edge of the 2-by-4 inch board.
Cut 2 more jack studs the same length as the vertical rough opening for the window and nail into place inside the king studs within the window opening.
Repeat the process in Step 4 for the base plate to measure and cut a header for the window. Nail into place on top of the jack studs within the window opening.
Measure the distance from the top plate to the header and cut enough cripple studs to have a standard 16 inches on centre spacing between them the length of the window opening. Nail these into place and you have completed a framed window opening.
If you are working with an existing wall rather than a new wall, remove the wall covering and expose the studs in the wall. Then you may proceed with the instructions above.