Drill holes in the steel plate at the four corners large enough to accommodate your screws.
Measure the steel plate when positioned at a 45 degree angle using the protractor. The distance between the front of the plate and the point where it touches the ground is the base length. The distance between the top of the plate and the ground is the leg length. Combined, the leg and base will make up two sides of the right triangle that will be the frame of the bullet trap.
Cut all boards with 45 degree angles using the miter box and saw on one end; on the opposite end, a 90 degree cut should be used.
Assemble the bullet trap frame with screws.
Attach the steel plate to the bullet trap frame using screws.