Screen-frame stock can be purchased in most home centers and is fairly inexpensive.
Begin by cutting the metal frame-stock using a hack saw and miter-box (Image 1). To save time and ensure uniform lengths, you can cut two lengths of stock together at the same time. Cut the equally long top and bottom pieces together, then the two identical side-pieces. The lengths of frame can be cut straight with no need for miter cuts since the corners of the frame are joined together with L-shaped corner clips (Image 2).