I did the Miralax prep with the Dulcolax pills and started it at 3:00 the day before the procedure (my procedure was at 8:00 the next morning). They gave me printed out instructions and there are also websites with advice on how to prep for a colonoscopy, including the best things to eat in the days before and such. There are also charts to let you know when you're done and your colon is all cleaned out (based on the color of what you'll be passing).
I seem to recall that I mixed up two quarts of Gatorade and Miralax and the instructions were to drink a certain amount every 15 minutes until it was gone. I also had to take a couple of Dulcolax. At first nothing was happening, but then suddenly it was!
My advice: Once you start taking the mixture, sit on the john and basically don't get off until you're done. Get some books and a radio. Because once things start moving you won't have time to get to the bathroom. Also get some disposable undies like Depends and wear them while you're prepping, just in case. That's what I did. If your prep goes like mine, you'll be passing mostly liquid and it will happen very suddenly with no warning.
My prep had no flavor except for the Gatorade (I used the lemon lime flavor). Miralax is flavorless.
Since it's your first, I'd suggest calling the doctor's office and ask for a detailed, exact schedule on when to do everything. If you're not cleaned out enough they won't be able to do the procedure and you'll have to reschedule and do it all over again, so you'll want to make sure it's done properly.
Good luck!