Well, you need a facility, liability insurance, at least a few volunteer helpers and you need to advertise...local publications, flyers in feed and tack stores and local stables, Craig's List. You will have to plan the program and decide on fees to participate or monitor. If it's all day you may need to think about food. People get hungry. Maybe a local 4-H club could sponsor and you split fees? Or let them do a hot dog booth for income to their club as long as no one runs afoul of concession permits and all that sort of thing.
And it seemed like a simple idea to start with, huh? : )
Good luck with this project. A successful event could bring in clients. Let us know how it goes.
If you ever find yourself in a fair fight, it's because your tactics suck. ~ Marine 1SGT J. Reifinger