How do I make an amended tax return? I found some W-2's in the mail that I need to submit

If you have already filed your tax return you need to first wait and see if your return is accepted or rejected. If rejected, you can go back and add the additional W-2's, then re-submit your return.

If accepted, you need to wait until you receive your tax refund and then Amend your tax return using the Amendment Form 1040X. 

We recommend waiting until after you receive your tax refund to Amend because if you file the Amendment prior there is a chance that you will delay your tax refund and you wouldn't want to do that.

Here is an FAQ with additional instructions on Amending your 2016 tax return.

https://ttlc.intuit.com/replies/5114064