Cut a piece of fiberglass screen a few inches larger than the opening. Starting at the top of the opening, lay the screen across the top channel track. Using a roller knife, evenly push the spline ($21 for 300 feet, The Home Depot) into the top track channel groove, fastening the screen into the groove as you go. Continue pushing the spline into one side track, then repeat on the opposite side, ensuring that the screen remains straight as you work. Insert the spline into the bottom channel track last.