Start with a prescription diet from your veterinarian. Multiple studies have found soy contamination in the over the counter allergy diet brands. Now if you dog isn't allergic to soy, this is no big deal, but since the only thing they've looked for so far is soy, you really don't know what else might be in there too. Reading the label is NOT enough to avoid certain ingredients.
Now take heart, this is only for the first 3 months. The idea is to use a diet known to be contamination free (same independent studies found prescription diets were free of soy) for the diet trial, and you can find out just how good your dog can be. You can then use this as a baseline for choosing an over the counter food later.
When choosing a prescription diet, you want to use a protein source your pet has NEVER had before, fish, venison, egg, kangaroo, and rabbit are the major choices. You also want to avoid ALL other food items during this time. Throw away all your pets bones, flavored toys, treats, and flavored medications (you will need a topical heartworm preventative). If you mess up, and your dog gets something they shouldn't, you start over from day one. It takes months for the inflammation from food allergies to subside, and it only takes a very tiny amount of the food to start the process over again.
I have a food allergic dog, he is allergic to beef and chicken. You can PM me if you have more questions about him and what has worked well for us
OH, wanted to add that homecooking is absolutely an option as well. If you decide to home-cook long term, I'd recommend getting a veterinary nutritionist to help you balance the diet. Homecooking requires a protein source and a carbohydrate source your dog HAS NOT been exposed to before, and may require some creatitivity.