Your extension will give you until October 15, 2019 to timely-file your 2018 federal tax return.
Online and mobile app: When you're ready to finish and file your return, simply sign in at TurboTax.com or the mobile app, reopen your return by selecting Take me to my return, and do whatever you need to do to finish up.
CD/download software: All you need to do is open (continue) your return where you left off.
The bottom line is that you don't need to do anything special to finish a return for which you filed an extension. When your return is done and you're ready to file, the File section will walk you through the filing process, whether you want to e-file or paper-file.