Unroll a sheet of tar paper on the roof, cut it to the appropriate length with a utility knife and staple it along the seams to the roof's plywood sheeting. Repeat this process until you have covered the entire roof with tar paper.
Measure the width of the roof and note the number in inches. Place the tape measure across a piece of the sheet metal and make a small snip with a pair of tin snips at the appropriate point. Mark a straight line with a permanent marker and straightedge and cut the sheet metal to the correct size with the tip snips.
Set the piece of sheet metal on the tar paper along the edge of the roof. Tar paper typically has lines to use for aligning the roofing material, but if the tar paper you use is void of lines, use a chalk line to make a straight line across the tar paper. Align the sheet metal with the line.
Hammer the sheet metal to the roof with roofing nails and a heavy hammer. Roofing nails have rubber gaskets directly beneath their heads that form a seal to prevent water from leaking through the nail hole. Place nails at 1-foot intervals around the perimeter of the sheet metal.
Repeat the process of measuring and cutting a second piece of sheet metal and affix the second sheet above the first sheet so the second sheet overlaps by 1 to 2 inches. Nail the second sheet into position and repeat this process across the entire roof.