Cut pieces for a 6-foot by 12-foot frame. Cut two side pieces of 3 inch by 4 inch angle iron 12 feet long with a metal chop saw. Cut two pieces of 3 inch by 4 inch angle iron for the front and back ends. Cut five pieces of 2 inch by 3 inch angle iron for cross braces. Miter two ends of the long rails and both ends of the front end and weld those together with the 3-inch angle up to hold floorboards. Make sure the corners are square. Cut small triangles of scrap steel and weld across the bottoms of the joint for support.
Weld the back end piece with the 3-inch angle down so that the flat side is up. Make sure the corners are square. Brace this with short strips of scrap steel welded at an angle across the joint. Weld the 2 inch by 3 inch cross braces with the flat side up at 2, 4, 6 and 8 feet from the trailer front. Brace these joints with diagonal sheet steel straps; braces where axle springs will attach will need extra support.
Cut two pieces of angle iron 81 inches long for a triangular tongue. Weld these from the side rails just in front of the first cross brace behind the front, angled to fit into a trailer ball hitch cap; the angle of the hitch cap will determine the angle of the tongue pieces. Weld the hitch cap into place. Match the hitch cap to the hitch ball on the towing vehicle.
Locate the axle about the middle of the load center. Place the axle so that either 60 percent of the trailer is in front of the axle or the axle is just behind the load center. Set the frame on the axle temporarily to see what works best and mark locations for spring hangers, which come with the axle. Weld those hangers into place; they are u-channel steel with holes to hold bolts for the springs. Weld extra diagonal gussets of steel to the side and cross members at points where spring hangers attach.
Set the frame on the axle springs and secure it with large bolts that go through the spring hangers and holes in attachment points on the ends of the springs. Cut 4-inch sections of 4-inch u-channel steel and weld to the outsides of the rails at 2-foot intervals, for stake holders. Put five brackets down the sides and two on the front.
Finish the frame by making a front support leg. Weld u-channel across the bottom of the yoke behind the hitch ball cap, with the open side down. Cut an 18-inch piece of smaller u-channel to fit inside that brace. Drill holes through both pieces at both ends of the smaller piece. Attach one end with a bolt, washers and nut. Fasten the other end with a large spring clip to hold it up when not in use.