Measure your patio area (length and width), and decide what style of aluminum awning you want -- solid panel, lattice or other variation. Also decide whether you want to attach it to the wall of the house (if the patio abuts the house), or if will be free-standing. Then figure your square footage (length times width) and order enough aluminum material to fit. Many aluminum patio roofing kits or awnings come as kits, complete with support poles and installation hardware. If you buy separate components, decide what kind of support and framing poles you want (aluminum, steel or wood) and buy those, with hardware.
Erect corner posts, if your patio does not already have them. If you attach the awning to the house wall, you can either place posts abutting the wall, or nail a ledger board (a 2- by 6-inch board) on the wall to support the aluminum framework. Sink posts into the ground at the corners of the patio, in holes as deep as about one-third of the height, and secure them with concrete. Or, mount pole plates on the concrete patio -- these are square or round holders with flanges that screw or bolt into the concrete patio. If your patio is bigger than your aluminum panels, add center posts for support.
Frame your aluminum cover, around all sides and, as needed, in the center. Use wood framing (2- by 4-inch lumber) if your exterior posts are wood, or use aluminum rails, which are box-like tubes that can be joined at corners with right-angle connectors -- the tubes slide into the connectors, which then mount to the posts with screws. If you buy an awning or roofing kit, poles and connectors will come with it.
Cover your frame with aluminum panels, screwing the panels into the side rails and posts. Start at the bottom and work up, so that panels overlap at the top seams. Make sure there is a slight slope away from the house so that water will run off and not collect on the aluminum roof. If you need center posts, you can make them taller, so the aluminum awning slopes away on both sides; then cover the peak with peak caps. Some solid aluminum panels come with interlocking sides, so one panel slides into the next. You will need ladders and help to install roofing panels. Use tin snips or a metal saw to cut panels to fit, as needed.