We are trying to get one going around here. The first thing you need is a suitable location with large enough arena, stalls (if necessary), water, parking, overnight accomodations (if necessary), and horse waiting area/holding pen.
We have started designing hazards in our heads. We found the photos on the Cypress Keep website helpful. Their ADT is in April, maybe you can go watch/participate if you aren't too far from Plant City, FL.
As Lori said, having access to a driving club for help is good. Volunteers will be necessary. As we get going, hopefully we can put together a "how-to" manual for the ADS.
Myrna