In the Billing tab, select 'New' then 'Invoice:'
Type in a client name (which will autofill from your client list), then a date for the invoice to be due. Select the terms of payment, and add items:
To add line items that pop up the same every time, follow this QUICK GUIDE.
Add client notes (that will be seen by the client) as well as Terms and Conditions. You can then choose to 'Send Invoice' (which will send to the client immediately) or 'Save as Draft' (which will allow you to come back to make edits and send the invoice later.