How do I start? Do I just eat once a day??
Ease into it! If you currently eat breakfast, lunch, AND dinner, start by pushing breakfast back by a couple hours. Eventually, phase breakfast out entirely. That would put you at something like 16:8 (16 hours fasting, 8 hour eating window). Eventually, push lunch back and work toward an 18:6 schedule.
At the same time, focus your meals on more whole foods (start removing processed foods and added sugar). As your meals become more nutritionally balanced, weed out all the snacks.
Happy to answer other questions as you get started, if you want to provide more specific stats about yourself (i.e. height and weight, starting weight / current weight / goal weight, what you want to accomplish with IF).