If you require a really primo bluing job you might need to find a gunsmith, but I get good results using a method I arrived upon through trial and error.
First, I have ready a roll of decent paper towel, like Bounty. Some 0000 steel wool torn into several cotton ball sized pieces, a couple of bottles of Birchwood Casey Super Blue, something to apply the bluing with, such as small pieces of soft cloth. I cut about thirty 2"x 2" pieces of flannel from an old shirt or sheets. I use alcohol to degrease because there is always some handy, but there are better things out there, I'm sure. I also have several pair of surgical gloves on hand, and a can of WD-40 with a small rag.
If you are rebluing and need to remove the old blue first, do that. I use Naval Jelly I buy at our local Lowes. My local gun shop sells rust and bluing remover in tiny bottles, but it's basically Naval Jelly.
It's very important to degrease. The bluing method I use will overcome a small amount of oil, greasy fingerprints, etc. But the better the job I do of getting the piece clean before I start, the easier things go.
Here is how I blue a water jacket. The water jacket is a real challenge. The hardest thing to get an even blued finish on is, predictably, a large smooth unbroken surface.
First I wipe the jacket thoroughly with an alcohol soaked rag. I also use the small alcohol wipes, like the nurse uses to prep a site for an injection. They're great for touchups as I go.
I pour some bluing in the bottle lid or small dish. I don't work out of the bottle. It takes two hands and will just slow me up. Also, when you get to the steel wool phase, any iron that comes in contact with the bluing will degrade the solution and you don't want to contaminate the whole bottle. With a gloved hand I soak a 2x2 cloth in bluing and start at one end, making long (12") strokes. The bluing will cause the metal to oxidize on contact. As it oxidizes the oxide begins to make the bluing inert. The rag will pick up some of the oxidant and make the bluing on the cloth inert. Keep getting a new cloth and dipping it in fresh bluing.
I rub the bluing on as if I were polishing brass. Firm pressure and scrub it on. Pressure helps keep the application even and reduces the spotting. As I move along, wiping the bluing on with one hand, I use my other hand to wipe the bluing dry, replacing the paper towel often.
The bluing really wants to streak and spot. Don't let it sit or puddle. You want to control the depth of the reaction - keep it as consistent as possible. It gets darker with each coat, so don't worry about getting it dark on the first pass.
At this point the bluing is streaked, uneven, and looks like hell.
After I have the whole barrel jacket covered with the initial coat, and dried, I get a piece of 0000 steel wool and repeat the process using the steel wool instead of a cloth. This is where I will even out the finish. I keep both the steel wool, bluing, and the paper towel fresh, and keep it moving. The steel wool oxidizes (turns dark) quickly. You will notice that when you use fresh bluing with fresh steel wool you get a good reaction or oxidation of the metal. Once the bluing and steel wool have been used you quickly begin to get no reaction at all.
NOTE: Be sure to check some out-of-site part of your gun to make certain that the steel wool won't mar it. I have never had it do so, but I always check anyway.
With a little practice, you will get better at applying and drying. Applying and drying. Use longer rather than shorter strokes, but don't get ahead of yourself. The bluing should never dry on its own. It should be wiped off. Wiping wet bluing spreads it as it dries and helps keep it smooth and streak free. If you have an area that is really uneven, put a little elbow grease into the steel wool.
After you have gotten the finish about the way you want it, look the piece over carefully. If there is a spot that simply won't take the blue, get an alcohol swab and clean it again, and touch it up. The steel wool will allow you to blend in small areas.
Water will stop the oxidization, so when I'm satisfied with the finish, I either run water over the piece if it is small, or use a damp cloth. If there are any areas where water can get trapped, I use an air hose to dry it. You can also heat the piece in the oven, but you invite rust. I then spray it liberally with WD-40 or Remington spray gun oil, then wipe it clean with a cloth. Finally I coat it with Break Free gun oil. Sure as hell, the oil will reveal a spot I missed. No problem. I clean the area with a swab and touch it up.
I wouldn't practice this method on my best firearm. I can't guarantee
you will get the same results I do. I'm not a gun smith, and for all I know
I may be the only one on the planet who applies bluing with steel wool, but
it works well for me. (Note: Since writing this I have discovered that there
is indeed a bluing method that employs the use of steel wool, so there is
precedence for this) Not professional quality, mind you, but good for a novice
do-it-yourself job. Good luck!
Addendum: Since writing this, I have added heat to the process when needed. As with browning solution, bluing will react more aggressively if the metal is hot, say 150-200 degrees. When I run across a piece that seems to resist taking as deep a bluing as I would like, I heat the metal in the oven - a toaster oven is great for the small stuff.
Disclaimer - I'm not a chemist and I don't have a clue what this stuff will do to you, cold or hot. My advice is to stay away from all bluing solutions, bore solvents, oils, Parkerizing chemicals, paints and guns altogether. You should be collecting stamps or Beanie Babies, and you can tell your lawyer I said so.