After you file

here's another way.

you must have a paper copy of the return you filed (or a transcript).

 

The simplest way is to prepare your correct tax return (using TurboTax for example, or other means) and print it.
Then lay the paper printouts of both returns (correct and original) side-by-side.
You now have all the information needed to prepare Form1040X by hand.
It's pretty straight-forward.

The blank form in fillable PDF format and its instructions are available at www.irs.gov.
Mail to the same address you would mail your original tax return.