If an estimate you've sent to one of your clients is accepted, invoicely will automatically convert it to an invoice for you. In some cases, however, you may want to manually convert an estimate to an invoice. In order to do this, please navigate to Estimates, open the estimate in question and then choose "Convert to invoice" from the top right Actions menu.
You will then be taken directly to your new invoice. Don't forget to hit the orange "Save" button when you're done.