I've done many rifles, pistols (alas, no revolvers) and shotguns. There are basically two processes, one involving heat and the other requiring significant humidity.
The most popular 'hot' process uses Birchwood Casey's Plum Brown. I build a fixture to hold the parts during the heating process and use a butane torch to heat the metal to about 260 degrees, or where water drops sizzle away quickly. The Plum Brown solution is applied with a large cotton shop swab (Brownell's sells them). Yes, the fumes are pretty nasty, so good ventilation is advised. The keys are cleaning/degreasing the metal and keeping it hot to apply the solution. Several coats may be required depending on the finish desired, but the whole gun can be done in a day.
The 'cold' process is best illustrated by Laurel Mountain Browning Solution; it comes with excellent instructions that should be followed to the letter, so I won't go into details here. Again, cleaning and degreasing well is important, although not so much as with the 'hot' process. The key with this method is high humidity; you apply the solution and let the parts sit in a very humid environment to form the rust. This process does require several coats; after the first coat you will think you've screwed up and ruined the parts as it looks terrible, but press on and it will soon get much better. Because the process requires at least 3 hours of setting each coat it can take several days to complete the process.