Watch the dog's reaction when you fire from a distance while they are having fun. Preferably when running through birds. It is really important that you watch the pup when they hear the report. If they don't appear anxious you can fire closer later. If they do you need to back off and let them build there confidence.
Also, take the pup everywhere you can--expose them to everyday sights and sounds while you are around.
It is almost as important to introduce the dog slowly to the field. Some dogs thought to be gun shy are actually field shy.
Getting dropped in a strange place with new smells and sounds, sights can be overwhelming for a dog that has not been out of the yard. If you can introduce gun fire around an older dog known to the pup that is not gun shy. Young dogs will look to older dogs to see how to react when they see or hear something new.

Shoot a 45 over the heads of the entire litter. Pick the one which runs toward you rather than away from you.