Do this between 18.00 the first evening and 18.00 the next evening.
Do this in a day without very much activity and when you maybe can stay at home or even better, in an open space in nature.. Please have a toilet on close distance as you would probably go to the toilet more:-)
Begin the first evening: stop eating after 18.00 hours.
You can drink lots of water, fresh juices or smoothies and go on a no-caffeine policy (no to black or green tea or coffee, yes to herbs teas for example).
Eat the next day only vegetables, salads and fruit as breakfast - or a big green smoothie!
Drink lots and lots of loved and filtered (or even better, ideas here) water, you can try it with lemon and even honey if you wanna, or go the whole way and mix it with chlorella and other green stuff..