one option would be to get their AGI for 2017 so you can efile. if not, mailing is a little more complicated. You must attach any withholding forms such as W-2's. Then make sure you mail to address in TT instructions. Use a delivery method where you get proof of delivery. Over the years I worked for various CPA firms and they all had situations where mailed returns never got to the IRS. Even, on occasion, delivery services have lost the packages. (cargo and passenger planes do crash, traffic accidents occur)
if you are getting paid to do this, there is a completely new set of issues including the fact you should not be using TT.
