If you want to manage & add "Invoices" you need to click "Invoices" section from left sidebar there you see your all estimates list and if you want to create new estimate then click on the "Create New Invoice" button from top.

✓ Step1: Add Invoice: When you click on this button you will see a form structure like invoice. Here you must select your customers & products. You can also add Tax & discounts for this invoice.

✓ Step2: Save Invoice: After creating an invoice it will be saved as a Draft so you need to approve this invoice and You must record a payment for this invocie otherwise it will be show unpaid invocie untill you not added any payments.

✓ Step3: Send Invoice: After record a payment now you can send a invoice copy to your customers.