Design your metal patio roof; measure width and length of your patio and decide how you want your roof to run. You can make a flat roof (with a very slight slope to let water drain), a slanted roof which falls away from the house or a peaked roof, which works well if your patio is big enough to require center posts between the corners. Any style can be built with either galvanized steel or aluminum panels.
Install your posts and build a roof frame. Wood posts are easiest --- dig holes a depth about a third of the length of the post, sink the post and fill the hole with concrete, making sure post is vertical and plumb. For a flat roof, make posts the same height; for a slanted roof, make one side slightly longer for a slope of at least one inch per foot; for a peaked roof, make center posts longer than corners. Frame the outline with 2 by 4 board. You can also use steel or aluminum posts, with framing of square or rectangular tubing connected at the corners with angle connectors and then screwed to the posts.
Cover your roof with steel or aluminum panels. For flat or slanted roofs, start at the bottom and work up, overlapping panels about 2 inches. Attach panels with galvanized steel or aluminum screws. For peaked roofs, run panels from edge to peak and cover the top with a peak ridge cap. Overlap panels at the seams; some galvanized aluminum panels have interlocking seams, with one panel sliding into the slot of the adjacent panel to eliminate overlapping. Cut panels to fit with tin snips or a metal saw.