Michael Pollan’s advice is sound. “Eat food” means eat real actual food, not fast food, not over processed food (Cheetos), not nutribars, but actual food you can look and identify with your eyes, food your grandmother would be able to recognize. “Not too much” means keep an eye on the proper number of calories you need to eat and stay within that (or simply don’t have second portions and make your first one smaller), luckily when you eat more veggies the fiber makes you feel fuller and there are less calories per amount of food. And “Mostly plants” means just that, eat fruit and vegetables as much as you can, choose an orange versus a fruit cup, an apple versus potato chips. Plants contain antioxidants, fiber, vitamins, water, and nutrients.
The reason you should avoid processed food is because each stage of processing generally takes some amount of nutrients out of the food whether purposefully or inadvertently, or adds in other less valuable content to bulk up the product. I believe in Michael Pollan’s book “In Defense of Food” he talks about this in relation to white and whole grain breads.
Always eat breakfast, try to cover the three groups (carbs, protein, and fat). I usually go for a bacon and egg muffin sandwich and a cup of coffee with a fruit later on in the morning.
If it’s a simple guide you’re looking for, I highly recommending simply eating more fruit and veggies, and cutting as much liquid calories as you can as those aren’t as valuable compared to drinking water and spending those calories on something with fiber.