Since I was doing it for the colonoscopy (scheduled for the next morning), I did the flush the afternoon before. I drank only clear liquids during the day and started the Vitamin C at 3 pm. Dr. Osborne had advised leaving 6 hours for the flush. After I started the protocol, I drank only water until midnight and then nothing until the colonoscopy was completed. The process seemed to work well–at least the gastroenterologist didn't complain!
If I were going to do the flush for its own sake and not for a colonoscopy, i would probably start in the morning on an empty stomach as they recommend and drink nothing but water until the C did its work and the “flush” started. Once it starts you stop the Vitamin C, but the flush continues for several hours. You have your “calibration” question answered at that point, but I would still drink only water until the bulk of the flushing was done. Each glass of water seemed to induce more “flush,” and I can't imagine what eating anything more would do .
I felt fine during the flush other than feeling cold. I never had cramps or anything uncomfortable. I made up each batch of Vitamin C as I went along, but in retrospect I would have made up each hour's worth ahead of time. I set a timer to make sure I didn't fall behind, and recorded how much C I was taking as I went along.
Good luck tomorrow! I'll be interested to hear how it goes and whether you are feeling any better afterwards.