Once you've created your sales order and registered as sent which saves the order within the system it can be transferred to invoicing.
Tick the box on the left hand side of the order and click Transfer to invoicing
A popup box will appear, this enables you to combine more than one order to an invoice. If you have several orders for one customer you can combine them to one invoice. If this is not required, don't tick the boxes and just click ok.
The invoice can then be found with Current invoices which can then be booked and emailed out to your customer.