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 tab'.
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. You can find out a detailed explanation of the quick bill screen via this article.
For this example, I clicked Go To Invoice & Do More. The below image shows the invoice. Information on the different selections here can be found in this article.