Cut your aluminum sheeting into the shape you wish using shears or a band saw; most street signs are generally diamond shaped or hexagonal.
Apply the letters for whatever words you intend to spell out on the sign. Many art supply stores sell letters with adhesive on the back. Otherwise, you can also use electrical tape to design the lettering. If you are planning to put the sign in a setting that has high amounts of traffic, it is best to use retroflective aluminum lettering so that the sign is visible at night.
Drill holes into the areas of the sign where you plan to use mounts for the metal braces. Affix four metal braces to the back of the aluminum in order to reinforce the stability of the sign.
Screw the sign onto a tall block of wood or metal plank, hammering the sign down in the areas where necessary.
Place your sign in the desired location, though you will probably only be permitted to put the sign out on your own private property if you have not arranged for permission ahead of time with the local authorities.