Decide which online environment you think will work best for the discussion you want to run. The Forum is probably the easiest and most convenient of the two, as students can access the discussion at any time, they can see the whole discussion in front of them, and it's easier to set-up and monitor.
Depending on which option you choose:
- Have students go to the Web Forum on their class page. For instructions on how to use the Web Forum visit: https://writing.colostate.edu/ceilidh/cheatsheets/ceilidh_info.htm
- Have students go to the online chat room on their class page. The instructions on how to use the Chat Room are provided prior to logging into the chat. If you choose this option, you'll have to consider assigning a time where people can "meet" in the Chat Room.
Depending on the type of discussion you want students to have, you might:

Post a prompt of your own to establish the focus and rules for the discussion.
Post a prompt that asks students to offer discussion topics of their own (which are relevant to the class of course).
Post a prompt that continues a line of discussion that was started during class-time.
In any case, make sure to provide a prompt that gives the students a clear sense of what your expectations are for the discussion.

No matter which you choose, make sure to establish clear expectations in terms of how often you want students to post/participate in the discussion, and what qualifies as participation (do they need to post something of a certain length? Or something that demonstrates they have given serious thought to the issue at hand?)