I did it myself. I kept a food diary, writing down everything I ate, and I eliminated the most common food allergens ... wheat, corn, soy, tree nuts, peanuts, dairy, and eggs. After two weeks of not eating any of those foods, I would eat one. You could have a reaction up to a day or two, but I'd usually have a stomachache or reflux within 15 minutes and then not poop for 3-4 days. You need to wait until all reactions have stopped before introducing another food. Since I reacted to almost everything, it took me several weeks to reintroduce all the foods.

If you eat packaged foods, you need to read the ingredients and write down everything on the label. People would always ask what I could eat, and it's actually a lot ... all kinds of meat and seafood, all the vegetables and fruits, plus grains like rice and quinoa. This is also a pretty cheap and easy way to eat. Grill or bake meat or seafood and steam or stir-fry veggies. Plain rice is super cheap (not a boxed mix, which includes soy and corn derivatives). If you have salads, be sure to read the ingredients on the dressing because they can include corn and soy derivatives like corn syrup, corn starch, dextrose, etc. There are about 100 different food additives that are made from corn.