Brian,
To get you going with this customization job, you should think about when you want this to be triggered. In this case, it may be best to trigger the activity creation when a user clicks the Convert to Customer button. If this is what you want, you should enter your actions into that button. I suggest reverse engineering some other actions in Method that perform a similar function to learn how to do this customization. One example could be the Schedule Follow-Up Activity option in the Edit Case screen. If you look into the Save Case, Activity, & New Activity action set, you will see the actions used to insert an activity.
If you need more tips let me know. As always, our in-house consultants or Method Solution Providers would be happy to help!
Jason