Cut the bottom corner of each angle iron at a 45-degree angle with your hacksaw. When laid out prior to welding these pieces will look like a mitered picture frame.
Weld one of the 2-foot pieces of angle iron to one of the 4-foot pieces of angle iron at the 45-degree miter cut. Weld them together along the 45-degree angle first and then up the vertical lip of the angle iron. (This results in a 2-foot-by-4-foot L-shaped assembly.) Repeat on the other two pieces. Lay out the two "L" assemblies so they create a rectangular box, and weld the two remaining corners together. This is the frame for the wagon box.
Space the six 2-foot-by-1-inch-by-1/8-inch straps across the bottom of the frame that you just made. Place them in 6-inch increments. For example, lay out the first strap 6 inches from the end of the wagon frame and so on. These straps will support the weight of items in the wagon.
Flip the frame over. The angle iron lip on the sides of the base will be pointing up. Fit the perforated metal piece into the base. It will rest on the straps that you just welded onto the lower lip of the angle iron frame. Tack weld the perforated metal to the angle iron base and straps in several places. This step completes the bottom of the wagon box.
Prime the metal with rust-inhibiting paint, and then spray with two or three light coats of enamel, allowing plenty of time to dry between coats. See manufacturer instructions for recommended dry times.
Follow any directions provided with the wagon kit to attach the axle mounting brackets to the bottom of the base. You will either weld or bolt them into place, depending on how they were manufactured.
Attach the padded handle from the wagon kit to the front axle assembly with the hardware provided.
Attach the wheels and axles from the wagon kit to the metal frame with the hardware provided by drilling holes with a cordless power drill.