If you do not feel well at any point, you must stop fasting. You may feel hungry, but you should not feel sick.
Drink water frequently throughout the day when you fast. Seriously. This is the cardinal rule.
To avoid refeeding syndrome, do not extend the fasting period beyond 14 days.
When you break your fast, eat gentle things for a day to get the machinery going again, like eggs or guacamole
Undertake 7 day fasts no more than once a month
Undertake 14 day fasts no more than every 6 weeks
Megan Ramos (works with Dr Jason Fung) recommends a teaspoon of salt per day, as well as magnesium supplements. She just puts Himilayan salt rocks on her tongue throughout the day, letting it get absorbed into her bloodstream there, instead of her digestive tract.
If you have a medical condition, make sure you do this with medical supervision. Your Doctor may want blood tests throughout.