Some people do eat anything and everything, others follow a plan, but more loosely than usual. You do have a calorie allowance to aim for. Results are supposed to be similar whether you use those calories on high or low fat food, so no need to eat the usual 'diet' junk. Not sure if high or low sugar has been investigated.
There is a temptation to binge a bit on the first few up days, although some people find they really don't feel like eating loads. And it seems to balance out!
as Teresa said, the first few downs are hard, but they quickly get easier. The diaries of the of the folks who have been doing this a while are amazing, and are the best explanation and recommendation!
hope it works for you