When you have an invoice, you can add due dates to payments through the Partial payments button located on the right side of your invoice. When you click on the partial payments button the Partial Payment popup will appear.
In the partial payment popup you can add Due Dates for payments on the left side. You can add an amount, a due date and a short description (e.g. First payment, Final payment etc.), when you click the Add button on the bottom the form and the due date will be added to the right side of the popup.
On the right side you can see all the current Due Dates for the invoice. When a partial payments gets paid by the client, you can set the due date to paid by clicking on the small checkbox in the first column of the list, a page will slide in where you can select the date when the payment was made.
When an invoice is partially paid, the status of the invoice will automatically be set to the Partial Paid status: