1) This maxim from Michael Pollan: "Eat food [1]. Mostly plants. Not too much." (I personally follow an adjusted version of this: "Eat food. Only plants. Not too much.")
2) Review daily nutrient requirements to ensure your diet is providing you with adequate macro (protein, carbs, fat) and micronutrients (vitamins, minerals).
3) A few months after adopting your new diet, get a high quality blood test that covers markers like iron, B12, etc., and make any required adjustments to what you're eating.
1: "Real food doesn’t have a long ingredient list, isn’t advertised on TV, and it doesn’t contain stuff like maltodextrin or sodium tripolyphosphate. Real food is things that your great-grandmother (or someone’s great-grandmother) would recognize." https://michaelpollan.com/reviews/how-to-eat/