Make a Frame
Use a metal chop saw to cut four pieces of 3-by-4-inch angle iron into 10-foot lengths and four pieces into 5-foot widths. Cut 45-degree angle miters on all pieces. Weld these pieces into two 5-by-10-foot frames, with the 3-inch angles up. Use a square to make sure they are square.
Cut six 18-inch pieces of 2-by-3-inch angle iron. Weld four of them upright on the outside of the four corners of one frame with the 3-inch angle facing in.
Set the second frame piece on those corners and weld it in place. Weld the other two 2-by-3 pieces upright in the center of the frame on each side for supports.
Add Axle and Wheels
Measure the leaf springs on the axle, from one mounting hole to the other (these are attached to each end of the spring) and mark those spots on the bottom of the frame. Weld one spring holder at each location. Match the spring holders to the axle; front holders will be longer than rear ones.
Cut two 5-foot sections of 3-by-4-inch angle iron and weld across the bottom of the frame just in front of and behind the spring hangers. Set the frame on the axle springs and secure it with a bolt through the spring hanger and the mounting hole in the spring. Fasten with a lock washer and nut.
Buy wheel rims and tires to match the lugs on the axle wheels. Install them with lug nuts; buy these at an auto supply store if they do not come with the wheels. Don't attach them until all welding is done.
Make a Yoke and Drop Gate
Make a yoke for the trailer hitch ball cap by cutting 2-by-3 angle irons to go diagonally from the front leaf spring brace support to the front of the trailer. The trailer hitch ball cap element will determine the angle and length of those sections. Weld the yoke to the frame at the side rails, where it crosses the front brace and to the hitch ball cap. Weld a cross brace just behind the hitch.
Floor the trailer with 2-by-6-inch wood planks; 10 should cover the trailer. Measure inside the frame sides, front to back and top to bottom, and cut expanded metal flooring with a metal saw to fit those spaces. Cut another piece of metal flooring to cover the front section. Weld those pieces to the frame.
Make a back gate of 2-by-3 angle iron, two pieces 18 inches high and two pieces 5 feet wide. Weld that frame together. Cut expanded metal flooring to fit and weld it inside. Weld gate hinges to the bottom of the gate and the rear brace. Weld a chain on each end of the gate to support it when it is dropped. Weld a gate latch on either side of the gate and end posts to hold the gate upright.