Determine the size of the finished canvas frame. Calculate the length of the wood pieces you need by subtracting 1/2 inch from each of the sides and 1 inch from the shorter side's length for the cross braces. Decide how many cross-braces you want for the frame. A good option is one cross brace every 18 inches.
Measure the length of each wood piece and mark it on your wood. These measurements must be precise, even a 1/8-inch error will leave your frame lopsided.
Cut the four frame pieces and as many cross braces as you decided to add. The cuts should be square, made at precisely 90 degrees to the length of the board.
Spread the tarp out on your work surface or the floor to protect your wood frame from nicks or marks that would show through the canvas. Set up the pieces for the frame by resting them on their narrower sides. Overlap the ends of the rectangle such that each piece overlaps the next on one end and is overlapped by a piece on the other end.
Lay a flat corner brace on one corner of the frame. Mark the holes for the screws and drill the holes on your marks. Screw the brace into place. Attach the brace on the opposite corner next, then on the last two corners.
Mark the holes for two wood screws to go into each corner of the frame. The screws should go down through the overlapping piece of wood and into the cut end of the piece butted up to it. Mark all four corners of the frame, making sure that the screws are at least 1/8 inch from any of the edges so that the wood won't split.
Drill pilot holes on the marked points and countersink two screws into each corner.
Mark where the cross braces will go along the longer side of the frame. Mark both sides of the frame.
Slip the cut cross braces in at the spots you've marked, checking that both sides of the frame are sitting squarely in the frame. Drill pilot holes on the marked points and countersink two screws into each end of the cross braces through the longer frame piece.
Sand over the corners of the frame and over each screw so no splinters work their way through the canvas. If any of the screws are countersunk too deeply, use wood putty to fill the depression.
Remove the flat corner braces, if you are going to mount and stretch the canvas immediately. If you are going to store the frame for a while before mounting the canvas, leave the braces on for storage to help keep the frame absolutely square.