I missed my tax due date for 2018 so I didn't file it in time and now I need to send it in by mail. How do I do so?

When you printed your tax return, TurboTax should have printed a sheet of filing instructions telling you what to include and where to mail it. 

If you don't see them, here's what to do:

Make sure you sign and date your return. If it is a joint return each spouse must sign it.

Include copies of all W-2s and any other forms that had federal income tax withheld.

Include all the forms that are part of your tax return but not worksheets. TurboTax prints the forms for filing when you use that print command.

Make sure you send federal and state separately if you are filing both. They go to different places. 

Here is how to find where to mail your federal return: https://www.irs.gov/uac/where-to-file-paper-tax-returns-with-or-without-a-payment?_ga=1.31357439.1770841720.1470007833

You would have to look on your states web site for where to mail your state return: https://ttlc.intuit.com/community/refund-status/help/how-do-i-track-my-state-refund/00/25571 This is actually for refunds but it might help you find where to mail the state return.