Measure the size of your window and it's position relative to the wall, recording the distance of the top of the window from the ceiling, the sides from the nearest side wall, and the bottom from the floor. Sketch out your frame wall design, and include an opening in the framing for the opening, plus 3/4 inch on each side.
Construct your stud wall from 2-by-4-inch boards, including the opening for the window. Nail the top and bottom plates for the window opening into the nearest studs on either side of the opening with 10d nails, then place the side members of the opening between the top and bottom plates and toe nail them into position. Cut vertical studs to fit between the top plate of the window and the top plate of the wall proper, and space them in the gap at an 8-inch interval. Toe nail each board into position.
Measure the width of the top and bottom plates of the window opening. Cut 1-by-4 boards to match the distance. Place the boards onto the top and bottom plates and nail them in place with 6d finishing nails.
Measure and cut 1-by-4-inch boards to match the vertical distance between the boards you've just placed. Put them in position on the sides of the opening and nail them in place with 6d finishing nails.