Painting metal cabinets

How can I paint old METAL kitchen cabinets?

It's really not that different from any other type of painting.  I assume you are going to brush as opposed to spraying.  I would take a fine grit sand paper and lightly sand, then clean with a liquid sander, then apply a coat of high quality primer (tinted the same color as the finish coat), very lightly sand and clean after this primer drys, then apply the finish coat.  Hopefully, you can get by with just the two coats.