Views:
Either in Order Manager, Invoice Manager within the order, clicking invoice new, or Invoice New button on the menu.  
 
 
Explanation:

Order Manager: New Invoice will create a copy of a specific order and allow changes to be made regarding things that may have occurred between the time the order was placed and the time the order is ready to be invoiced to the customer.

Once the order has been saved, the New Invoice button will appear either on an Order Manager or a New Order screen. Selecting this button will create a copy of the order in a new tab as an invoice. Notice as of this time there is no invoice number assigned.

Invoice Manager: In a New Invoice or an Invoice Manager, pull an order in as an invoice by selecting the order number within the Invoice screen. Notice as of this time there is no invoice number assigned.

Within the order you want to bill and invoice click the invoice new button.  You may get a message that items are being updated with cost and quantity changes based on what costs from the vendor were recorded.   Click ok and verify that all things billed out are the right quantity and costs.  When this is saved it will save as invoice number zero.  To invoice it and assign the invoice number, click process invoice.
On the menu click the New button in the invoice group.  This will open an invoice window. Enter the order number you wish to invoice. The screen will populate with the order information allowing changes that have occurred between the time the order was placed and now invoicing the order. Notice there is not invoice number assigned.

Regardless of which option you choose, an invoice number is not assigned until the invoice is processed with the Process Invoice button.  This posts a ledger entry to update sales and costs.  This also records a customer receivable, a payable to the appropriate sales tax vendor and account as well as a payable to the appropriate commission vendor (salesperson) and account.