The key thing is to be using the easiest "cleansing" medicine. It's a prescription they give you to take the night before. I used SUPREP. It's 6 oz of fluid you dilute with 10 oz water and consume over a period of 15 min, followed by 32 more oz of clear fluid over the next 45 min - for that I used gatorade. There are two of these 15 min / 45 min doses for suprep, and I kept sipping gatorade afterwards.
Anyway that amounts to 48 oz of fluid spread over an hour (done twice), which is kind of a lot, but other preps involve up to 64 oz of fluid in an hour (done twice). The suprep tastes unpleasant, but I always followed it with a gatorade chaser. And I didn't quite make my 48 oz in 1 hour deadline, but I continued drinking gatorade.
I once tried mixing the suprep fluid with cranberry juice but it was a mistake. the fluid will ruin the taste of whatever you mix it with.
You will be visiting the toilet repeatedly. Don't try to hold back. The prep fluid dehydrates you, and gatorade or similar is the best thing I know for getting a little sugar and minerals back into your system. But it will run thru you fast.
I've had two colonoscopies, and I try to schedule them first thing in the morning so that I'm doing my prep later at night and not disturbing or being disturbed by family members. When I got there in the morning I was a little chilly and a little weak, but they put me under warmed blankets with an IV drip to get blood sugar & minerals back up.
During prep you'll be constantly "going" to the bathroom and it will eventually be purely liquid. It's normal.
Be honest with them if it's still happening the morning of the procedure. They ideally don't want it to, but you want the procedure to go right so being honest is best.
You're statistically set up to come out of this just fine. This happens every day to tons of people and they're fine, too.
I'd recommend wearing a buttplug for the week prior to the probing so you don't feel it when they probe you. Just walk around work with it in you. If anyone asks you why you got a bulge back there just tell them you are studying hard for a upcomming exam.
My husband followed his preparation instructions to the letter. It cleaned him out thoroughly, made the job of the medics easy and resulted in perfect images and diagnostics. The anesthesia meant he couldn't remember the procedure, except for the very beginning, so there was very little discomfort and zero pain.