File as normal. Log into your account and finish entering all your info. Then continue through the File tab, through the 3 steps all the way to the very end and hit the big orange Transmit Returns Now button. You can efile until Oct 15, 2019. After that Online will shut down and you have to print and mail your return.
If you made a payment with the extension,
If your extension payment did not automatically show up on 1040 Schedule 5 line 71 which goes to 1040 line 17 you need to enter it. So make sure it's not already there before you enter it!
You can type estimates paid in the search box at the top of your return and click Find and it will give you a link to Jump To the screen where you enter Extension Payments.
To enter the amount you paid with an extension (including state extension) go to
Federal Taxes (or Personal for desktop H&B version)
Deductions and Credits
Then scroll way down to Estimates and Other Taxes Paid
Other Income Taxes - Click the Start or Update button
Next page second section Payments with Extension
Either Visit All or Click Start or Update by the extension you paid