Lay the piece of sheet metal on a flat work surface such as the floor. Turn your table upside down and set it in the center of the metal.
Trim away any excess metal that extends more than 6 inches beyond the table top. Use tin snips to complete the trimming process.
Draw a straight line with a marker from the corner edge of the table top, extending upward to the end of the sheet metal piece. Draw another one extending from the corner to the side of the sheet metal. Repeat this on all four corners of the table top. Cut along the lines drawn at the corners using tin snips.
Hammer the sides of the sheet metal upwards so it bends around the sides of the table top onto the bottom. Repeat this on all sides of the table.
Nail the sheet metal into place on the bottom of the table by driving nails through to the wood beneath. Use nails that are shorter than the thickness of the table. Flip the table upright to use.