You don't fill in the 1040X directly; instead, after TurboTax thinks you have filed (whether by e-file or by mail), the TurboTax sets a flag that puts you in amend mode when you re-open the return.
Any changes you make after that point will go to populating the 1040x, which you can print (along with the other forms that have changed) and mail.
Does that answer your concern?