Measure the length of the area where you want to build a knee wall. Cut two pieces of 2-by-4 to this length. These two pieces are the top plate and bottom plate of your knee wall.
Measure along the length of the top and bottom plate and mark off 16-inch increments. Count the number of marks to determine the number of studs you will need.
Measure the vertical distance from the attic floor to the underside of the eaves at the location where you want to put the wall. This is the height of your knee wall. Subtract 3 inches to account for the thickness of the top and bottom plates. Cut the studs to this length using a circular saw.
Lay out the knee wall flat on the floor. Place the studs in between the top and bottom plates so they are at right angles to the plates.
Assemble the knee wall by driving two 3-inch-long screws through the faces of the top and bottom plates and into the ends of each stud.
Stand the knee wall up in place. Check that it is vertical, using a 4-foot level.
Secure the knee wall in place by driving 3-inch-long screws down through the bottom plate into the attic floor and up through the top plate into the attic rafters.