Need some information on fasting...

Hey there all lovelies....I would like some information on juice fasting: the safe way to do so and how long would be healthy for me as a beginner...I've been a little leary of fasting, as I am quite thin and tall...don't want to lose the weight that I already have! I also have hypothyroidism...strange, I know, as I just mentioned I'm quite thin ( I did manage to gain some weight due to the thyroid problem, but like I said, I would like to keep it!)Any suggestions?

Many thanks in advance,
Eire