To create an invoice, you first need to open the job that you want to create an invoice for, which can be done by clicking Jobs on the top menu and then selecting the job from your jobs list. Once you're on the actual job page, click Choose an action and from the drop down select Create an Invoice. That will take you to the new invoice page, where you can add line items, taxes, and notes for your customer. You can also select the due date of the invoice: upon receipt, within 7 days of receipt, within 14 days of receipt, and within 30 days of receipt.
After you're finished, click the Create and Preview Invoice button. That will create a draft of your invoice, which can be edited, emailed to a customer, marked as sent, and marked as paid by clicking on Choose an action and selecting the appropriate option.