I have coffee in the morning, 1-2 cups, then just water the rest of the day. If you're fasting for true fasting effect (autophagy, etc.) then you can't eat more than 50cal or you'll "break" the fast.
If you're fasting for dieting/reduced calories, then having a protein shake (100-200cal) is OK if it helps you get through the day.
I'm lucky in that my body responds will to fasting, but you may need to ease into it - aim for not eating until 12pm (12-8pm), then push that later each week as you feel better until you get to 4pm or 6pm and are doing OMAD.
Also very important when fasting is food quality. OMAD is marketed as "eat anything you like and lose weight!", and while that may be true you'll feel like crap if the only meal you have pizza and ice cream and beer. Start with a salad, then get some good protein & fat in your entree, with a side of carbs. Dessert is OK, but it should be a treat - not a full serving/second meal.