Dig an 18-inch-deep hole at the four corners of the patio with a shovel. Measure the sides of the patio on the outside of the holes. Cut 2-by-4s to the length of each side with a circular saw and install them around the perimeter of the patio. Insert a deck screw at each juncture with the screwdriver attachment of an electric drill or an electric screwdriver.
Construct a frame to hold the corrugated plastic roof by cutting four 2-by-4s to the size of the patio perimeter then installing a deck screw at each juncture. Set aside the frame until you install the 4-by-4 poles to hold it.
Mix two bags of cement with water in a wheelbarrow, according to the package directions to get the proper consistency, then pour the cement into the first hole. Place an 8-foot 4-by-4 into the hole and use boards to keep it straight until the cement sets. Repeat this process for the other 4-by-4s at the corners of the patio and cover the holes with soil once the cement hardens.
Install the frame on top of the 4-by-4 poles. Use an electric drill or screwdriver to install screws at each intersection of the frame. Put corrugated plastic over the frame and nail it to the roof.