Yeah, I've had a few!
What happens when you go in is they will sit you down add a cannula and take your pre glucose reading.
Then they give about 75gms of glucose.
They take readings every fifteen minutes unless they give you a continuous glucose monitor.
The whole process is to see how your response to the glucose reacts with your blood.
It will show initial insulin response and how high your spike is, then to see where it is after two hours. It will also show secondary insulin response.
They may take other bloods for other things like c-peptide.
It is a standard test and the doctors can gain a lot of information.
They use glucose because of its the quickest way to put glucose into your blood, because it is the stuff everything you eat and drink is converted to for your energy.
I always took my e reader and my music to while away the time.
My glucose tolerance tests lasted over four hours because they weren't checking for diabetes, it was to see if I went hypo, because my pancreas misbehave after carbs and sugars.
Hope this helps and it doesn't hurt at all!