I've never held a detailing conference, but it's pretty much the same as any other conference.
1) First, you need to advertise it to enough people to make it worth it. That's probably the hardest part.
2) You also need to arrange the space to have it at - will it be inside or outside? Will you be serving food? If so, you'll need a caterer and those prices could change depending on the number of people you have attend. The big draw of hotels or places that specialize in hosting conferences is that you can get food, refreshments, a hall, and waiters all from one source. Still, be mindful that anyplace will require you to have a headcount up to 2-4 weeks prior to the event, so you can't wait until the last minute. Make sure you have people RSVP with their admission fees to lock them in.
3) People want to learn something at a conference. It's not enough to just have people meet up and sit around and talk. Call manufacturers of product lines or local detailers with good reputations. Ask them to speak on different aspects of the profession such as what products they use on which jobs, proper use of rotary tools, etc. The manufacturers are usually happy to send out a representative to hawk their specific line of products and give demonstrations, samples - it's like a live infomercial for them. You could get one major sponsor to offer products and demonstrations, or several sponsors to split the cost/effort and offer a variety of things.
4) If you are doing this on a large scale, you could also have a large number of product representatives come out and set up booths, so that you could have a small trade show at the event where people can network and possibly place orders. However, if you have no experience in setting up things you probably want to start small for this one.
I would recommend either hiring an agency to organize all this if you are trying to do something large, or make a half-day affair with one or two product reps, several demonstrations, and a couple of seminars if you are doing this all by yourself for the first time.
Finally, don't forget to advertise, or even gauge interest before you do this. The best planned conference in the world will flop if you have no one attending it!
Hope this helps - I've set up everything from my high school reunion to large scale conferences for the Department of Defense. I've glossed over a lot of the details here, but will be happy to answer any other questions for you. It takes planning, attention to detail, and patience, but you can do it. Just don't half-ass it or you won't ever be able to hold a second one!
Thanks