Prepare the work area by taping or covering anything not to be painted.
Take a sheet of sandpaper and start roughing the surface of the metal. The reason for this is that some paint won’t stick to smooth, polished metal.
Give the paint a good mix (or shake for spray paint); it should be one solid color, and thick and creamy.
Apply a thin first layer of paint to the metal. This first layer will absorb into the abrasions made by the sandpaper and really lock the paint to the metal.
Wait a couple of hours while the first layer of paint dries.
Apply another thin layer of paint on top of the first. This layer should fill in any missed spots, but mainly serves to reinforce and make absolutely sure the metal is sealed and protected.
Continue to apply layers as necessary. If the metal is really scratched from the sandpaper, there may still be abrasions visible through the first two layers of paint.
Reapply the paint every couple of years to keep the metal protected from oxidation.