Measure the height and length of the space available in the attic for your knee wall. If the knee wall will run parallel with the joists and rafters in your attic, you will also need to measure the distance between the rafters for additional supports.
Cut a pair of 2-by-4 lumber to match the length of your knee wall. These boards will form the horizontal top and bottom plates for your wall. If the final length of your knee wall is longer than your boards, divide the total length of the wall into separate sections of wall so that each section is the same length and cut your top and bottom plates accordingly.
Measure out boards to fit between the top and bottom plates of your knee wall. Make their length equal to the total height of the wall minus the thickness of the top and bottom plates -- this is typically 3 inches. Cut enough studs to install one support every 16 or 24 inches along the top and bottom plates.
Place the bottom plate on the attic floor and mark the positions for the studs. Measure the placement of the studs from the center of the bottom plate, spacing them 16 or 24 inches apart.
Nail the vertical struts in place using your hammer. Drive the nails through the bottom plate and into the studs. Position the top plate above the studs so that it is even with the bottom plate and secure it to each stud with nails.
When you install the wall, secure it to the rafters using nails. If your attic does not have a floor you can install 1/4-inch plywood across your attic joists. Anchor the flooring to the joists using nails or wood screws and anchor the knee wall to the plywood flooring using nails or screws attached to L-brackets.