I'm not sure about how to make freeze-dried fish flakes, if that's what you mean by 'fish food,' and it is not recommended that you try. Administering the required vitamins a fish may need in its diet is a hassle when you don't have at least some formulated diet to fall back on. As ReefAdvise said, crushed shrimp is an excellent choice for a lot of fish, but no single thing should be the one staple of a diet. It would be smarter to buy a variety of different foods, both fresh and formulated (there are many quality brands out there such as Omega One or New Life Spectrum).
I would suggest using a quality pellet or flake food appropriate for your type fish, and then supplementing the diet with other foods such as shrimp, spirulina, or mashed peas. Vegetarian fish tend to love softened zucchini, spinach, or romaine lettuce. Some fish (usually marine) like seaweed flakes or sheets. Goldfish, koi, and some others may enjoy oranges, lemons, or other citrus.