If you are a sysop, have dispatch privileges, or if your organization allows user maintenance scheduling...
You will see a link for making a maintenance schedule. (On the new schedule you would drag the +Maint bar to the schedule). General users can only make a short maintenance schedule starting at the current time. Sysops and dispatcher can make a maintenance schedule for any time and any length.
When making a maintenance schedule, you may have the option to to set a schedule message, resource message or squawk (depending on the configuration of your club). The schedule message will show in the description of the schedule. The resource message adds a message that can be viewed through the red ball link on the schedule to the left of the N-Number. The squawk can be seen in the squawk list or through the hazard symbol left of the N-Number.
At this time general users cannot edit maintenance schedules (even the one they just made). Sysops and dispatchers can only edit maintenance schedules when they are in dispatcher mode. Or, they can edit by right-clicking a maintenance schedule and selecting 'Edit'.