The quiz needs JavaScript
Without it, the short version is this: if you have never fasted, start at 12:12 — stop eating after dinner and do not start again until breakfast. If that is comfortable after a fortnight, move to 14:10, then 16:8 if you want to. Do not start with an eighteen-hour fast.
If you are pregnant or breastfeeding, under 18, take medication for blood sugar, or have a history of disordered eating, talk to a doctor before trying any of it.
The calorie calculator works without JavaScript-heavy steps and will give you a number to work with.