Peel shrimp, devein and set aside in cold water. Never leave peels on the shrimp. Chop onion, celery, and saute in butter until tender. This is going to be your roux. Don't let it burn but once it has browned sprinkle 2 tablespoons of flour and stir just like you are making gravy. I use fresh tomatoes that I freeze and put the tomatoes in (you may need to add water). Add all the seasonings bay leaves, garlic powder, etc. I use the Gumbo File throughout the process of cooking gumbo. I usually start by adding a tablespoon of File to this. I let this cook until the frozen tomatoes are thawed and everything is blended well.