How to attach a payment to the (correct) document?

Payments that are not attached to a document will appear in the billing section like this, a free-standing payment:
If a payment is attached to Proforma or Invoice you will see a small arrow under the document, indicating there is a payment attached:
Clicking on the arrow will open up the payment:
In both cases the steps are identical to attach the payment to the correct document.
Click on the More Action Button behind the Payment and choose Switch Proforma/Invoice. :
In the pop up screen that appears you choose the correct document the payment should be attached to:
and hit Save
The payment has now been moved from a Proforma and attached to the Invoice:

When a payment is attached to the incorrect Proforma or Invoice, or when the payment is not attached to any document at all, you can simply change this with the following steps: