Pick a niche diet (for example, keto-vegan) that you follow yourself. Prepare everything you plan to eat tomorrow and blend it into a paste with a blender. Divide it into as many portions as you have meals. Eat only your mix.
Based on experience, try to improve the formula for taste, smell, nutrition for the next day. It should be at least neutral if not pleasant. Keep on improving it.
Study human nutrition and figure out what the essentials are, regardless of the diet. Make sure your mix contains just what your body needs, but no more.
Figure out a way to extend the lifespan of your mix to a few days, so that it doesn't go bad too fast. This will likely be some form of dehydration.
Get a few friends who are on similar diets to try your mix and see if they would be willing to pay for the ingredients/work for you to prepare it for them. Would they try eating just your mix for a week, 2 weeks, a month?
Improve your product based on what you learned from your friends.
Find more people that would give your mix a try and commit to the diet for a test period. Your only objective at this point should be to produce a good product that people who are not your friends want to pay for.
Improve your product based on feedback and work on extending its lifespan further. Pelleting, dehydration.
Think about ideal packaging, brand, logo, etc.
Think about rules and regulations, company formation, etc.
Think about automating your production line. Machines, workforce, etc.
Think about marketing and distribution channels.