Only eat it if you make it.
There would clearly need to be a few rules, since tossing a Stouffer's Family Size Mac 'n' Cheeze in the microwave counts as cooking to some.
So to start the diet, all the food you make needs to start from items that are only one ingredient themselves. So you can have a sandwich, but you would need to make the bread. (Flour, water, yeast, salt, maybe a little honey?) See, bread has several parts, but I am willing to concede flour is 1 ingredient. Heck you could even have mustard and mayonnaise... you'd just have to make them.
Once you get a handle on things you could then eat things already prepared for you that you COULD make if you had the time or wanted to. Like frozen foods where you read the label and recognize every single thing as food. But not where the frozen food lists processed derivatives of food substances you have never heard of, or can't get your hands on as a home cook.
And that would be it.