You can read about the details elsewhere. I'll hit the high spots.
The doctor can't see anything if your intestines aren't clean. I was instructed to fast the day before the procedure. Sure, I could have a small amount of semi-solid food such as unflavored Jello but why bother. Starting at midnight, I ate nothing. I was free to drink clear liquids, coffee, tea, and soft drinks that had no red dye in them. In the late afternoon, I broke out the 'bowel prep kit' which I got from the pharmacy. The kit contained a plastic jug with a powdered chemical within, some flavoring packets, and 2 pills. The first thing I did was to take 2 pills. They didn't seem to do anything. Then I added enough water to the jug to create a liter of a flushing fluid. (Apparently the procedure has been improved - others tell me of having to drink a gallon of the stuff, not a liter! A gallon of anything would be very very tough to drink.)
An hour after taking the pills, I started drinking the flushing fluid at the rate of 8 oz every 10 minutes. 4 glasses total. No big deal. It doesn't taste bad but it is a little thicker than water and somehow 'wrong.' What I learned was that you chug it. The end of the glass never stops going up until the liquid is gone. And you don't just pour it in, you suck it in. Get it in and down as quick as you can. The reason is, if you stop, it is very hard to start again. Like I say, it's odd stuff.
Over the course of the evening I chased it with a few glasses of water and got the desired result. I wasn't on the toilet all night or anything like that.
Then, at 5am on the day of the procedure, I repeated the flush process with 4 more glasses of the stuff. By the time I was done, I was as clean as a that area can probably be. I felt as fresh as a country meadow. :-P
I was getting very tired of washing my hands.
The procedure itself was a breeze. First, I removed my clothing and donned a hospital gown designed to be insufficient on multiple levels. Then I sat and froze in an examination room for about 90 minutes. /fume
The next stop was the room where the procedure happens. I got an saline IV and answered some medical questions. The nurse produced the instrument which had a business end at least 120 cm long and about 15mm in diameter. It's marked every 10 cm, lol, for navigational assistance. She plugged this nozzley thing, which I assume had been previously sterilized, into a machine. When the time came, the nurse injected *something* into the IV. I got drowsy pretty quick. The nurse instigated some chit chat...
And I was done. I was awake and my wife was there. I don't remember going to sleep and I don't remember waking up. 20 minutes later I was in the car, and 30 minutes after that enjoying my first solid food in over 42 hours.
I woke up a little during my procedure, watched for a while on an LCD screen that was set up, felt some pressure and said "ow", and the nurse knocked me back out. No pain at all that I can recall.
So if you need to get it done, don't worry, it really isn't a big deal.