If you've already created a Quote, your items will automatically be transferred over to your invoice. However, if you just want to send the invoice straight away you can simply do so by clicking "Create Invoice" in the specific job page.
Here you will see "Create an Invoice" as the square to the upper right of the specific job page.
Once you've selected this option you add any costs you want to be reflected on your invoice.
There are items for time (e.g. labour or consultancy costs), for materials (e.g. inventory, spare parts) or other (e.g. parking, postage).
and once you have saved your costs you're all set to send to your customer.
Click "Create an Invoice" in the upper right corner of your screen.
After you've clicked through you can add as many line items as you want and choose whether or not to charge VAT.
You can set up your invoice by calculating price per hour/day, or simply by project cost. You can even account for cost of material if necessary.