There are three optional reminders that can be sent for an outstanding invoice.
Head to the Service Reminders And Email Templates Settings to edit them.
You can adjust the content of the message, how many days after the invoice date will trigger the message to be sent, as well as if you want to be copied in to any emails sent.
For the emails to be sent automatically, simply set the dropdown to "Yes".
Now any future invoices you produce using the software will automatically send out reminders to the customer.