Lay out the pieces for the base frame: two 1/2-inch pieces for the sides and three 1/2-inch pieces for the top, bottom and cross pieces. Make sure the edges are square and flush. Glue the ends of the top and bottom pieces of the frame and attach to the sides. Use 1-inch pins to toenail the pieces together. (The term toenailing refers to attaching the ends of two boards together by nailing at an angle.)
Once the base frame is built, apply glue to the back of the two 1/2-inch pieces that will make up the top frame (2-3/8 by 75-1/4 inches and 2-3/8 by 35-3/4 inches). Apply over the base so they sit flush with the outside edges. The horizontal beams should overlap the seams of the base frame, making it a stronger structure door.
Attach the 2-inch upper cross piece on top of the lower one.
Place the screen material over the lower portion of the door and staple it to the 1/2-inch lip of the base frame with a manual stapler using 1/4-inch staples. Use plenty of staples, inserting as far as possible so they'll stay secure over the long run. Repeat for the upper portion of the door. Once the screen is attached, remove the excess material with a utility knife.
Insert the diagonal cross brace (2-7/8 by 42-inches; long point to long point) into the lower section of the frame. Measure diagonally from corner-to-corner, and cut a square piece of redwood to size. Hold the brace in place and mark where the corners will need to be mitered to fit. Cut and install by toe nailing with 1-inch pins.