Draw circles on the cardboard of various sizes, ranging from about 2 inches in diameter to about 5 inches. Use a compass, or trace round objects like bottles, large coins or the insides of tape rolls. Position each of the circles at least 1/2 inch from any others to make room to draw the teeth of the gears later.
Create smaller circles centered inside each of the larger circles. These inner circles can range in size, as not all the gears have to have the same proportions. Each gear's inner circle should be spaced at least 1/2 inch from the outside, but otherwise, it's up to you.
Cut a small rectangle of paper board to serve as the pattern for each gear's teeth. Again, the exact size doesn't matter as long as you like the look. Make a few different sizes of teeth, but some gears can have the same size teeth even if the spokes are different sizes; this creates more variety in the proportions.
Sketch the teeth of the gears around the outside of each outer circle. Leave a space between each tooth about as wide as the tooth itself.
Cut out the cardboard gears and their inner holes using the craft knife on the cutting board or self-healing mat.
Lay the cutouts flat on a sheet of waxed paper. Work in a well-ventilated area.
Spray the gears with varnish; this helps the brass paint go on with a more shiny finish. Let the gears dry, then flip over and the spray the other side.
Paint the gears with brass spray paint.
Apply another layer of varnish.