Application Builder User's Guide: Debugging an Application and more specifically, 14.4 Accessing Debugging Mode
It's really not hard. You click debug, the page then refreshes and the debug value has been set to yes. This will cause debug to be generated for the "show" action of the page, and you can view that debug by clicking "view debug", selecting (most likely) the top entry in the report (which simply lists all generated debug reports per page). When you submit the page it's the same, but the type will be "accept" instead of "show".