Where do i sign my tax returns if filing by mail

Yes that is the NEW postcard size 1040.  Under the new tax laws, there are no more simple Form 1040EZ or 1040A's.  Everything goes on a little Form 1040 that has six extra "schedules" with it.  The new 1040 is 2 half pages but now print on 1 sheet with the Signature line in the Middle.  

The IRS has a new approved method to "sign" your tax return. The Self-Select PIN method requires taxpayers to provide their prior year Adjusted Gross Income (AGI) amount or prior year PIN for use by the IRS for authentication.

1. Sign in to your TurboTax Online return.
2. Select File and you’ll get to the [Your name], just (…) more steps and we'll file your returns screen.
3. Complete Steps 1 and 2 if you haven’t already done so, then select Start next to Step 3.
4. On the Let’s get ready to e-file screen, select File by mail, then on the pop-up screen make sure you select File by mail for federal and all of your states.
   • If the File by mail option isn’t available, you probably previously selected to pay your TurboTax fees through your tax refund. In order to file by mail, you’ll need to pay with a credit or debit card (or use the Free Edition of TurboTax). Here’s how to change your payment method from pay by refund, to pay by credit or debit card.
5. Once you have changed your payment method to credit or debit card, follow the onscreen instructions to print your return. If you haven't already paid any outstanding TurboTax fees, you'll be prompted to do so before you can print.
6. The mailing address should be provided in the filing instructions that print out with the paper copy you file. 

Not anymore for 2018.