It depends on what your goal is for the fast.
The first few days are the hardest. I do fasts for weight loss. So my basic principle was, as needed eat or drink salt, pure fat, ACV coffee or tea almost in that order.
Past day 3 or 4, all the cravings and hunger etc etc disappear and you can truly water fast.
Think of a weight loss fast as a ramp. The further you are up the ramp, the less you should stray else you'd fall and hurt yourself.
Now if it is a fast to cleanse, I'd almost say the same thing except your fast begins on the last time you eat anything but water.
If you're dry fasting, your fast obviously starts when you finish that last drink of water.