Personally I would ease into it if you’ve been eating a lot of junk and have not been fasting. Maybe Omad first with lower carb diet then a longer fast. As for supplements I take magnesium glycinate and nu salt. The sidebar has good info on sources and amount. I found the only way I could tolerate the taste was to put in vegetable broth (which some say breaks a fast but I personally am fine with it).
Skip on any fast paced physical activity. Learn how your body reacts on that long of a fast first. Personally, I just relax and move slow. You won’t faint if you have enough excess fat in your body. Don’t get up fast, don’t lift weights.
You’ve mentioned eating junk food for weeks now. You might experience withdrawal symptoms so the electrolytes might help you. They’re expected, I just try to meditate or watch fasting videos to get through the struggling period. I haven’t had electrolytes but on my second week I’ve added salt and sometimes baking soda to my water when I’m feeling nauseous. I’m on day 19 of a 21 day water fast by the way.