If you have lots of credit notes, you can assign them to invoices in bulk.
Click the "Bulk Assign Credit Notes" button at the top right.
You will see a list of all your unassigned credit notes, and on the right, a drop-down so you can select the member's invoice you want to assign it to.
Assign the relevant invoices then click the "Assign To Invoices" button at the bottom.
To leave a credit note, simply leave the "Select invoice" option selected and the system will ignore that one when you submit it.