I start when they are young or whenever i get them even if they are a couple years old.
Bang the food pan on the ground a little at first more latter. then feed
no gunshots at the kennel
do not take the dog to the skeet field and tie him out on the high house!!
i had to fix one this happen to. if he even saw a gun he would get goofy
(it took a long time and alot of patience for him to get over it)

Take the dog out let them have fun and learn to hunt. put the dog in lots and lots of birds. this takes time several weeks to several months.
teach the dog a hunting pattern, always let them find birds around you,and let the natural instincts go to work.
the dog will let you know when it is time to kill a bird for him.