There are a number of different types of patina, so the first step is to study and find out what you want. Fading, cracking, chipping, scrapes, etc. Here is an example of a fairly complex rusty chipping old paint patina.
You've basically got 4 main colors, white base, cracking red main coat, a dusting of red primer and a little brown rust here and there. The quality of the base coat in this case doesn't matter, in fact the worse the better. First an ugly base of white, the more chips and scratches the better.
Then a good coat of the top color red, and a dusting of red oxide primer over that (the red oxide primer is flat and dull, so it looks dirty/faded.
With a fine wet sand paper most of the red oxide coat is removed, leaving a little here and there. Next a very coarse piece of sand paper is folded and the edge is used to create the white detail of the cracking/chips/wear shows through. Then a little rust solution is added here and there to create rust spots in the chips. This solution is basically iron filings suspended in a sort of paint. In this photo the rust solution looks gray in some spots because it hasn't rusted yet.
To finish it off the fork is wiped down with a greasy shop rag which stains the white paint and gives the whole fork a uniform low gloss finish.