When you use Twitter to facilitate an online event there are a couple of things you need to do.
1. Set up a Twitter account.
2. Decide on a date and time for your event - probably an hour is a good length of time.
3. Decide on a hashtag for your event - Facilitating Online uses the hash tag #FO2011
4. Decide on couple of questions that will lead the discussions.
4. Advertise the event to your networks.
Here is a video that will shows you what a Twitter looks like.