My experience will probably be different from others as I did sort of a stable vs stable kind of showing and we also were put into a lottery to determine who rode what horse (you never rode your own horse, it was about testing your skill as a rider, not judging the horse's skill), but I assume the process is similar.
If we bring a horse (most of the time the person holding the event will supply the horses), we will drive up the night before to get the horse settled in and then we retire to a hotel. Get up about 5am, head down to the show grounds to meet everyone, check the schedule, and get the horses ready for warm up.
During warm up, everyone would pay attention to what horse they had been drawn to ride, just to get an idea of how the horse will act and ride.
After warm up is a lot of sitting around and eating snacks, and then once the show has actually started, everybody in the class about to go on will get on their horse and do a quick warm up (literally you get to trot the horse once around a round pen and then you get to jump over one fence at your showing height to get a feel for them. ) and then the show is on. Repeat til all classes have finished and then you go to receive your ribbons, clean up, pack up, and head home.
I made this very short but it ends up usually being a 14 hour event. I'm not sure of the process of shows in which you ride your own horse and what not, but this was just my experience.