I'm working on an old 12 ga shotgun and all I used was Birchwood Casey cold blue and cold blue paste.
1. Cleaned/scrubbed/removed old bluing (wasn't much left) with BC blue remover, rinsed in water, blew the water off with an air hose, towel dried then dried with a heat gun.
2. Rubbed parts in cold blue, warm up the part with a heat gun (don't cook it, just warm it up), let sit for 10 min or so then rinse in cold water, dry, lightly rub down with 000 steel wool, blow off the part with compressed air.
3. Repeat step 2 about 8 to 10 times. (This could take hours so clear your schedule.)
4. After you feel you have blued the part enough, coat it liberally in gun oil and let sit.
5. Come back the next day and coat the parts in gun oil again. I do this about 4 or 5 times.
6. When you are ready to assemble, wipe the parts down as well as you can so they aren't sloppy wet with oil but are still oiled.
*Note: You should be wearing gloves for every step of this process, until you get to the oiling, so that you do not transfer oils from your skin to the steel.