Suggest trying Brownell's at:
www.brownells.com
I think they offer FREE instructions about some bluing systems.
I assume you're talking about real rust bluing vs the bottled cold blues sold at Wal-mart.
Real rust blue is applied by heating the part in a tank of boiling water, swabing the chemical on, allowing it to work, "carding" it off with fine steel wool, and repeating the process many times to get the depth of finish desired. This is easy to do, but very time consuming, and does give a fantastice true blue finish.
Since it is labor and time expensive, few 'smiths offer it.
The standard "cold" blue is really a touch-up for scratches and small scuffs. It won't do a really good job of matching an original blue job, doesn't do a very good total refinish, and is NOT durable.
To do this type of finish, clean the parts with a GOOD solvent (acetone, laquer thinner, etc). Warm the part up (warm NOT HOT), apply the chemical with a Q-Tip or swab, and allow to work according to the directions on the bottle. Wash off in warm water. Lightly rub the part with very fine steel wool, clean with solvent, and re-apply until it looks OK. After the final rinse, coat with a good common oil, and allow to "age" for a day or so.