There are lots of different approaches to fasting, one common one is to only have one or two meals in a day. To do that:
Open the Plan Creator by tapping the + button and then choosing Create plan.
Tap on Meals
Choose the two meals you'd like to include (for example lunch and dinner)
If you'd like to adjust the calories tap on the calorie dropdown
You'll see a calculation at the top showing what your total calories add up to from the meals you've selected.