It's really easy to do, you just don't eat. As bling said, you lose about a pound a day, but if you are much overweight you may be pleasantly surprised and lose more than that, particularly at the start of a fast. Having said that, at the end you will probably put a few pounds back on, but that's fluid etc, so don't worry. Thus on average, most people agree about a pound a day.
Fasting also makes you appreciate good food in the long run and even and cures lots of illnesses. You may feel hungry for a few days at the start, but after that you won't. I have lost about 70 lbs since November 2007 doing various fasts between 7 and 27 days.