I had a barium enema and for prep I had to drink a gallon of Golytly. I stopped drinking it at the point when there was only clear liquid coming out. Then I went to bed.
The barium enema wasn't horrible. The worst part as I recall was trying to hold it in while on the table. But I laughed about it all which made it better. When it's over, you'll make a dash for the bathroom to let it all out.