Start simply with water boiled potatoes, then carrots, then slowly things like bread. My parents are doing a one week fast every year and this is what they do. It takes time to go back to normal food as the bacteria in your intestine need to grow again.

The first time you do a one week fast, do it with people who are used to do it. In Europe you can find a lot of associations/clubs/whatever offering a one week seminar to do it.