There are two methods to create a follow up task or appointment.
From the menu bar
Select a task or appointment and choose File > New > New Follow up Task.
For a followup appointment, select File > New > New Follow Up Appointment.
From the contextual menu
Right-click a task to open a contextual menu and then choose New Follow Up Task.
For appointments, right-click an appointment and choose New Follow Up Appointment.
If you do not see these options available in the contextual menu, you can add them. For more information check out How do I customize my Contextual Menu in Daylite?