Fasting days you restrict calories to 500cal for girls. You eat what you like within those calories, though you get more bang for your buck with veggies/salad, plant based foods.
Ideally the authors suggest one or two meals through the day.
An ideal fast is to fast from end of meal the night prior, until breakfast, then fast 12 hours until teatime. In that theory, you get 2 or 3 x 12 hour fasts in over the 36 hour period (if you don't eat breakfast the next day until 7am).
I tend to fast until dinner time, though some days I have a tuna salad at lunchtime like I did today.
ETA - I believe the weight loss benefits come from the restricted calories, regardless of what time you eat them. I believe the other benefits are more reliant on stricter fasting.