Follow-ups can be set in two places
From within a specific order that you are working on for a customer. Go to the "Manage" tile in the header. Then go to "Order Settings", click on "Schedule Future Follow-Up" and set up when you are going to follow using those settings.
2. Under the "Customer Invoice" tile within the "Checkout" area of an order, there is an option to email the invoice to your customer. Click on the "Email Invoice" option to send a business email to your customer. At the bottom of the email area, is an option for a follow-up to be set as well. Once you click "Send Email" the follow-up will be created.