Waterfasting to lose 150 pounds and saggy skin...18 years old

Hello Everyone,
I am 18 years old, 5ft 6 inhes (1.68 meters/168cm) and 19 stones. After my exams, i am plannng on starting a Water Fast from then on (Late june) until the end of july (maybe in to august if i can go that long) and i was wondering A) Roughly how much weight would i lose? and B) Would excess skin be a problem?
I already know to have plenty of rest so most of the energy can go in to healing, only drink water, don't take supplements (i tried and i felt nauseous for the rest of the day), I will develop symptoms i.e. headaches, muscle cramps, nauseous, white coating on tongue, to end the fast when i feel intense and true hunger, get up slowly to prevent feeling dizzy and if i do feel dizzy just sit back down until it passes. I've read up on water fasting.
Weightloss is the main reason i'm doing it. Also, when i lose the weight (about 150 pounds) how to i combat excess skin? I have dark stretch marks around my upper abdomen and light stretch marks on my sides. Would or or how do i get rid of saggy/excess skin?

Thanks :D Tweet