Agree with all of the above methods. My method has been with the aid of a partner but it can be done alone using a field like Zeb said above. Try to have a bird involved if at all possible. Food/treats are a good 2nd option. Since I can only give my experiences, I'll share what I've done with my dad for 5 dogs below:
Find somewhere that has tree cover if possible to help deaden the sound. We used our long driveway at our cabin, but somewhere with tree cover will do. Start about 200 yards away from the person shooting the gun (.22 pistol or smaller caliber). Have yourself a pocket of GOOD treats. Hot dog chunks work well. Have the shooter shoot every 30 seconds or so. When a shot goes off, treat. Next shot. Treat. If they seem to care less about the noise, move a little closer. Shot, treat. Continue slowly moving closer. Treating with each shot. Until you can either A: Get close enough to the shooter that you are practically next to them while they shoot and you treat. Or B: The dog shows any sign of hesitation or concern about the shot.
Have yourself a method to notify the shooter to stop. Whistle, phone, etc. A big thing to remember also is to be CALM while doing the whole process. Your dog will pick up on you being nervous and may get scared wondering why you are nervous about the noise. Another thing I've done is talk to them when they shots go off. When a shot goes off saying something like "Find the bird. Good girl/boy" in a happy, playful tone.