http://www4.uwsp.edu/psych/dog/LA/castle2.htm
This is a common use of the training collar, aversion training with a twist. The twist is that you not only want the dog not to eat stray piece of food, but that you also must train that its O.K. to eat under certain circumstances.
While its appropriate to train a bird dog to NEVER chase deer, its not too good to train a dog to NEVER eat. They tend to get real thin and then die.
You have to decide beforehand what you want from your dog. Do you want the dog to only accept food from you? (Better not ever go out of town or on vacation). Do you want the dog to only eat from a special dish? (Better not ever lose or break that dish. Better not take the dog out of town and forget that dish).
Im going to assume that you understand the concepts behind the use of the collar and that you have desensitized the dog to wearing it.
Do this training before his mealtime, when you are sure he is hungry. Pick a day when you have no commitments for the whole day.
Put the collar on the dog with the highest possible level of stimulation.
Put some tasty treats of a type that you dont normally feed in the yard in a place where you can clearly see them. These should be smelly treats, that even you can smell. You MUST be able to see them so that you can give a correction to the dog for approaching them.
When the dog approaches the treat and is about 6" from one of them, give the dog a one full second stimulation.
Do not give less than one second, and do not give more.
The dog will probably avoid THAT treat in that particular location. You should be in the house watching through a window. It works best if the dog does not notice you watching. The confrontation should be between the treat and the dog. If the dog notices you, it may come to believe that you had something to do with the stimulation and will only not eat found food when you are present.
Sooner or later, and this may take some patience on your part, the dog will notice another piece of treat and will head towards is. When the dog is 6" from that treat give a three four second stimulation.
That treat in that location is probably now history. Your dog will probably ignore it for years.
Now is when it gets difficult. It may take an hour before the dog finds and goes for the third treat in the yard. If the dog finds that treat and goes for it, give the dog a five second stimulation. He needs to have a third correction, no matter how long it takes.
Bring him in and feed him from his bowl.
Repeat this tomorrow, with the same treats in different places. Pick up the old treats. Ideally he should smell the treat and veer away from them. If he does, bring him in and feed him.
If he goes to eat one of the treats, give him a five second stimulation.
By now, for sure he should be smelling the treats but veering away from them.
Repeat this the next day.
If he still avoids the treats, feed him from this bowl in the house.
Repeat this with another type of smelly, flavorful treat. Repeat this with all the treats you can think he may like.
Repeat this in all environments he is likely to find himself; the park, the front yard, etc., virtually anyplace you may go.
See why this is so hard? Dogs rarely generalize training to different locations.
Theres much more, but Im sure you get the idea.
Mostly people are only concerned with their yard so that makes it a bit easier. You have to try ALL treats. If you forget one, your dog may think that this one is O.K.
This should get you started.