You can manually raise an invoice via a client record by clicking on 'Actions' and then 'Add Invoice' or by clicking 'Add New' on the 'Account'.
Both instructions above will bring you to the 'Quick Bill' screen.
It is at this point where you will have to select what you want to add to the invoice. For my example, I am adding the stock item 'Foam Roller.' To do this simply type beside '+Add New Item'. Once you start typing it will bring up items which contain the letters/words you are entering and you then choose what you want to add.
Once this has been added you are brought to the screen below. It gives you different options to proceed with.
For this example, I clicked 'Go To Invoice & Do More.' The below image shows the invoice.