Changes in your diet a few days before the commencement of the fast has no bearing on how the fast will go.
Eat organic fruits and vegetables in raw form, which may help. In order to really benefit, eat raw for one month, then fast.
Fasting with water only means just that, no chewing gum, mouth washes, toothpaste etc. no supplements and vitamins.
It is simple to fast, but harder to deal with detox symptoms. Basically you need to rest, keep warm and drink distilled or filtered water as per thirst.