You can also start with it, when you add a cheat day in a week or after 2 weeks. That's how I started and on the cheatdays I ate less junkfood/sweeties/etc. because my body gets used to the healthy stuff. Meanwhile it becomes a cheatmeal on this day, where I don't eat more junkfood, sweeties etc. than this. Try it on this way! It is also a training for being more disciplined!