There are a couple of ways to get some practical experience:
1. Practice at home. Practice putting combos together, "hearing" the music, and queuing. I know I sound like a weirdo at home in my workout room queuing the invisible people behind me -- but it really does help. It gets you into the habit of thinking a few counts ahead while your body is still doing something else.
2. Ask an instructor to be a mentor for you. Sometimes they will let you teach the warmup or a combo or two as well as giving you valuable feedback. So then you are only responsible for a part of the class instead of an entire hour!
3. If you are interested in a job as an instructor -- just ask around at different gyms in your area (YMCAs and community REC centers too). Most of the time they need people -- or at the very least need someone as a sub on occasion.
4. Jump in head first

This is what happened to me and I wouldn't necessarily recommend it. I was in a class where the instructor tore her calf (ouch!). We were friends and she didn't want to stop the class -- so she asked me to finish it. Holy Moly -- I was TERRIBLE

No prepared combos. No feel for the music phrasing. I must have done a million lunges and basics (it was a step class).
At some point, you just have to take the plunge. The first time is a little scary but it all gets easier with time. Practice, practice, practice. Some classes are better than others. Sometimes you just have a brain meltdown and have to gather your thoughts and continue. I've had it happen to me and I've seen it happen to other instructors too. I'm assuming it happens to everyone ocasionally.
Good luck! It's exciting!
Shonie