You print out your 2018 PDF copy and mail it in....or if you have a paper copy in a folder (you should print one every year), you could send that in by mail.
......signed-dated...including any W-2 forms and any other tax forms that show Federal withholding
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If you don't have the PDF copy of other more recent years, you should print them out NOW since TTX drops access to one year as each year passes.
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IF you can't find the PDF or paper forms for your 2018 taxes, then your only recourse would be to re-prepare your 2018 taxes using the IRS's webpage of download 2018 forms and instructions...and then print/ fill them out by hand. (or go to some local professional in your area). Software for 2018 is no longer available.