For guys: eat nothing for 16 hours, eat you daily calories in the next 8 hours, then nothing for the next 16. Most people eat between 10-6, noon-8, or 2-10 depending on schedule.
For girls: last I heard, use a 14 hour fast followed by a 10 hour eating period.
During the fast part, no calories are allowed. No gum with 1 calorie. Just water, tea, black coffee, etc.
Some people do a 24 hour fast, then eat whatever they want for the next 24 hours. But that always seemed a bit silly to me and difficult to control.
Anyway, it's very simple once you start doing it. I generally eat between 1 and 9 and really only start getting hungry around 12/1230 so it's not exactly a struggle.