Yet another simple idea that I either never thought of or had forgotten (same end result) : pinner said: If you clamp them to a carpenters square you not only get a true 90 degree angle you leave your hands free to drill the screw into place.
Right Angle Braces - Wood Mag Tip of the Day For each pair, start with a square of Baltic-birch plywood and cut it in half diagonally. Then cut notches for the clamps, above. You can vary sizes to suit larger or smaller projects. The braces help