Galvanized roofing sheets come in 42-inch-wide pieces that overlap one another by one to two inches. Measure your roof's length, including overhangs, and its width (measure the left side of the gable's width, including overhang and multiply that by two). Convert the length of the roof to inches by multiplying the length by 12 inches. For example, if your roof length is 20 feet, will have 240 inches. Now subtract 80 inches from the 240 inches (the two end pieces minus 2 inches of overlap) and you have 160 inches. Now, divide that by 38 inches (one sheet's width minus the four inches of overlap, two on each side), and you need 4.2 sheets, plus the two end sheets, equaling 6.2 sheets, to lay across the length of the roof. You probably can't buy a fraction of a sheet, so you'll need to buy seven sheets and cut one or have it cut for you.
Work facing the gable of the roof and from right to left. Measure from the top edge of the roof 42 inches. Do the same at the bottom and then snap a chalk line across both marks. Repeat on the other side of the roof. This will be the reference line to which you will lay your first sheet.
Lay the first sheet of roofing on the chalk mark, beginning at the bottom of the roof and working up -- the sheets will either be 10 or 12 feet long, so you will need to overlap the proceeding sheets as you work your way up the roof in a single row. Apply two-inch galvanized screws into the valley of the metal pattern. You will work from the bottom to the top of the roof until one row is complete and then move to the left for the next row. Lay the next sheets overlapping the previous row to the right. The raised sections interlock. Finish attaching all the panels with screws. Attach the ridge cap, usually available in 10-foot sections, onto the ridge of the roof. This will seal of the roof at the ridge point where the two sides meet at the top.