First off, we'll start with cleaning the old headlight. This was painted black, but in some spots rust was showing through. So, first step is to clean all the rust off and make the surface nice and smooth (no bumps).
I did this with an angle grinder and sand paper. Just do it careful, because ones you shave it off, you can't put it back!
After the grinding off the lamp, I'll start prepping this for painting. Since the surface is already nice and course (for the paint to adhere better to the metal) I only had to clean off the dust and use some soap to remove residual oil from, for example, touching the lamp (those are bad for your paint layer).
While I let the headlight dry in the sun, I start with the cleaning of the chrome parts. This is done by pouring cola over the affected area and rubbing it consequently with the silver paper (use the shiny side). After a few rubbings the chrome will almost look as new!
For more information on that technique, visit this instructable: Chrome polishing - mythbusters style