How do you file just your state return

You can't e-file your state return if you filed your federal return by mail.   It is a security measure.   You can add a state return and file it by mail as well.  When you sign in to an already filed return, scroll down and click Add a State; it is at the very bottom of the page.  

Did you use Turbo Tax to prepare the federal and mail it?   You can only efile state if Federal was efiled and accepted from the same account.  So you will have to print and mail state.

If you didn't use Turbo Tax for federal, There is a way but you first have to fill in the federal and pay for federal for it to flow to state.  You will have to mail state.  See,

https://ttlc.intuit.com/community/filing-and-paying-taxes/help/can-i-prepare-and-file-just-my-state-return-using-turbotax/00/26408