Summarize the following:
The perimeter pieces are the 4 pieces of metal that form the rectangle for the trailer’s base. For this trailer, use 2 length pieces 6 feet (1.8 m) long and 2 width pieces 4 feet (1.2 m) long. Either buy 4 pieces cut to this length, or buy a longer piece and cut it to size yourself.  For this trailer, you’ll need 20 feet (6.1 m), or 240 inches (610 cm), of steel total for the base. Buy enough to complete the job. If you’re building your trailer to different dimensions, then adjust this step to meet your needs. Use a protractor and measure a 45-degree angle into both sides of each perimeter piece. Make sure the angles on each base piece are facing the same direction. Then use a power saw or miter saw and cut the angle into each piece.  Miter saws are best to use here because you can adjust the saw to the angle you want to cut. This ensures you get a proper, 45-degree angle. Most power saws can cut metal, but be sure to use a blade designed for metal. Always wear goggles and gloves when cutting metal. A welding mask would be even better, because cutting metal produces sparks. Once the angles are cut, lay out the trailer perimeter. Work on a flat surface, like your driveway or garage floor. Lay out one of the 6 ft (1.8 m) pieces. Then fit a 4 ft (1.2 m) piece into each corner. Lay down the last 6 ft (1.8 m) piece to complete the rectangle.  Push the pieces together so there is as little space between them as possible. If you aren’t sure if you’re working on a flat surface, put a level on the floor and check. the perimeter pieces together. With the perimeter formed, fasten the pieces together. Use a blowtorch or similar welding tool and weld along the corners of the rectangle. Weld all the areas you can reach first. Then let the metal cool down and flip it over so you can reach the bottom.  Make sure there are no openings or holes in your weld spots. This keeps water out and prevents rusting. You can rent welding equipment from hardware stores, or buy it if you prefer. Wear a welding mask, thick gloves, and a leather apron while you weld. Never touch metal right after you welded it. Wait for everything to cool down first. Use the same type of steel beams you used for the perimeter. Measure the width between the 2 longer sections of the perimeter. Then cut a piece of steel to that length. Fit it in between the length pieces to make sure it fits snuggly. Don’t worry if it’s a tight fit or if you have to press the piece in. It should fit tight so it welds better. Measure the trailer lengthwise and find the halfway point. Mark this off with chalk. Then fit the support beam into this spot and weld it in place.

Summary:
Get four 2 in (5.1 cm) x 3 in (7.6 cm) pieces of rectangular steel tubing for the perimeter. Cut a 45-degree angle into the end of each perimeter piece. Arrange the base pieces on a flat surface. Weld Cut a support beam for the center of the trailer bed. Weld the support beam at the halfway point of the trailer length.