Cut a piece of cardboard to match the size and shape of the sign. Buy a precut piece of aluminum or have a carpenter or someone at a hardware store cut out the sign for you. If you want a special shape, take the dimensions and a sketch of your sign for him to follow. Only cut the aluminum yourself if you are experienced with a metal-cutting saw.
Cut a duplicate piece of cardboard for the back of the design. Draw the desired shapes and lettering on the cardboard with a marker. Cut out the shapes with a utility knife. Do the same for the back of the sign. If you want the back and the front to be identical you only need to make one stencil.
Place newspaper under the sign. It is best to work outside or somewhere with good ventilation while using spray paint and spray adhesive. Tape down the sides of the newspaper with masking tape if it is windy.
Lay the stencils flat on the newspaper with the back sides facing up. Be sure you do not spray adhesive on the wrong side, especially if you have lettering on your sign. Wipe the aluminum clean and let dry. Spray a light, even layer of adhesive on the cardboard, then stick it to the sign.
Spray-paint the sign. Use a paint that works well on metal. You can use any color, but get one with a bit of shine or reflectivity so it resembles a real street sign. Let dry for one minute then apply a second coat if desired. Let dry another minute then remove the stencil. Let dry for one hour.
Spray-paint the other side of the sign. Let dry for one minute, remove the stencil, then let dry one hour. When both sides are completely dry wipe off any excess adhesive with a wad of newspaper.