1. Go to the Clients page. 

2. Click on the client you want to set up an invoice/payment reminder for. 

3. Click on Reminders and then click on +Add another reminder

By ticking off the box next to text: Automatically send payment reminders for this client's invoices, any invoice that is created for that client will have the auto-reminder switch turned ON by default. The reminder can be switched OFF on an invoice-by-invoice basis. 

You can add multiple reminders, all customized to your preferences. By clicking on the mail icon next to due date, you can customize the email your clients will receive. 

4. When you create new invoices for that client, you will have an option to turn the reminder ON/OFF