From the Membership Menu by selecting the 3rd item down 'Sundry Invoice' and click on insert.
Through the membership details screen you can only produce a invoice for the member you are looking at. Whereas from the membership menu you can create several invoices consecutively for different members. Whichever way you choose to create the invoice the screen that appears is the same, as shown below. The system will create the invoice number for you.
Just complete the information and amount as required (remember that this will only post to the general ledger if you are fully integrated)