Well, at least you have experienced fasting. Ideally you need to rest completely and stay warm. But since you are in college you will have some stress. It will be harder in the latter part of your fast. If you start tomorrow 20 dats will end on a Sunday. Monday morning you will break your fast. Fasting is not be used for weight loss. You will gain it all back after you start eating. Real loss will come if you do not eat processed junk and meats.
If you want to succeed in fasting with water only, start with an enema or drink Epsom Salt before commencement of your fast. Drink warm water as per thirst. Also don’t take supplements, brushing with toothpaste, no chewing gum, etc.