I have personally tried fasting to lose weight several times. I have found that it is something you have to be very careful about.
Of course, you really should never fast meaning not eating or drinking anything at all. There is too much risk of that leading to an eating disorder and it is not very likely that the weight loss will be lasting because as soon as you start eating you will likely gain back all the weight.
If you are going to do a water fast you need to drink a lot of water to make sure your body stays hydrated, but I do not suggest that you do a water fast for any more than one day at a time. I have only done a water fast a couple of times. I like eating too much to go without any food at all! When I did choose to do a water fast it was when I felt very internally heavy because I had eaten too many heavy foods for too long a period of time.
I have also tried juice fasting and that is a little bit easier because your body is still getting nutrients from the juice. The problem that I have with juice fasting is that I don't care for the taste of a lot of juices so it is hard to do this for any more than 3 days, but it does leave me feeling lighter and healthier.
For me, the best fasting method has been a combination of lots of water, a few fruit or vegetable juices and a little bit of food, but mostly raw fruits or vegetables. Even thought this is not a true fast because you are still eating, it works to lighten up your body and eliminate the heaviness that you feel from consuming unhealthy foods.