I had a colonoscopy and endoscopy in July this year.
TBH the thing that worried me most was having some of my dental crowns knocked out but I came through it and had my gallbladder out this week and we still have a full tooth status
The Prep- they'll give you a diet list. The day before you prep you need to be careful what you eat. I had poached eggs and white toast.
On the day, I think I could have bland breakfast (toast and my last cup of milky coffee) at 7am. Then it's fluids (I had black coffee, clear apple juice, lemonade)
The prep I had was two tablets then two drinks (it was a cloudy lemony drink, it goes hot, use cold water to dilute then neck it with a straw. It's not bad really)
My bowel movements started at 12 noon right through to 2am. It's like when you have D&V but withoutout the vomiting. But because there's no underlying illness it's not so bad, less cramping .
The actual procedure: I have had 2 dc with gas and air and all my 4 wisdom teeth out with a local anasethetic but I wan't to be sedated because I really didn't want to be in the room IYSWIM.
I could feel pushing of the endoscope but not gagging.
I could feel a pinching of the colonoscopy but nothing worse.
It's a bit like when you're asleep and someone wants to wake you up, you feel "Naff off and leave me alone"
I took a wind relief prep with me but I didn't need it.
I took spare pants and a sanitary towel but there was no leakage.
We got a taxi there and back, but on the way I felt floaty but not weak as I thought I'd be.
Bland food for a while, lots of fluid. I got a print off to give to my GP (Ulcerative Colitus as well as the gallstone which is now gone)
I did buy some Sudocream to put on my bum, but it stung worse. I'd use plain Vaseline I think if I had to do it again.
I went through a whole roll of soft loo roll, and took over the downstairs loo (no-one else at home, IIRC the DC came home at lunchtime on prep day, it was their last day of school)
Anything else, just ask 
They were very efficient, there's no embaressment (except when you need to pass all the gas before you can have coffee. But that cup of coffee was The Best ever 
And the worry about what they'll find. Of course it's a huge worry. They didn't tell me much when I was there (the bits they remove take 6 weeks) but they were looking for colitus and that was what they found.
Good Luck.