First thing to do would be to message your clinician and ask them if there is anything in specific they like their organizers to know or provide for the participants. Some clinicians have expected attire and/or tack (thinking of Lucinda Green, who famously wants bats and martingales for all riders) that need to be communicated to participants.
Have organized, clear parking with ample space to tack up. A communal place for riders to get on (mounting block, etc) if they are shipping in is nice too, as well as wash/water amenities if you are in a warm climate. I agree with signage and a helpful person directing traffic.
Structure your timeslots by competition level, not rider’s confidence/perceived level. Get to know your participants ahead of time; you may find you need to move participants around depending on horse/rider pairs. Is the horse green to the level, is it a new partnership, is the rider a pro on a greenbean or is it a timid re-rider? Etc. It is no fun being the only BN rider in a Novice-Training group and it’s also no fun being ready to move up to Prelim and being lumped in Novice.
Bring a spare bat, spurs, martingale, and lunge line to wherever the riding ring is.
Advise your riders to warm up 10-15m prior to start times. At their assigned time they should be warmed up enough to pop over their first fences (assuming jumping).
Building on above, allow 10-15m break between sessions so clinician can go to the restroom, grab a snack, or even pull a rider aside to discuss how the session went, etc.
If you have auditors make sure they have chairs and that the riding ring is accessible and the audience is set up near the clinician. If the clinician is to be mobile, give them a loudspeaker.
Coffee and snacks are very nice but not expected/required. Many clinics here put on a bagged lunch and riders are encouraged to watch other sessions.
Hope you have fun. Good luck.