There is a great video about Follow-Ups:  

You can manage your follow-up settings in the My Follow-ups section in your Settings (access through the menu icon of 3 horizontal lines in the upper right corner of the screen).

There are four tabs in the follow-ups section:

Follow-Ups: Assign default or custom follow-ups to tasks throughout Fiitfu. You can make follow-ups active or inactive on this tab.

System Follow-Ups: To get you started, Fiitfu comes with a set of default generic follow-ups that you can customize and use.

Custom Follow-Ups: Add your own custom follow-ups that you would commonly use. Then assign them to the appropriate section under Follow-Ups.

Funnels: Create your own custom funnels to group follow-ups together.