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 follow-up 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?