Keep in mind, this return will have to be printed and mailed as well. Prior year nor amended returns can be e-filed per IRS regulation.
You will want to include any W-2s or 1099 forms that have federal tax withheld, if you made any changes to those sections. Basically, any new information that you entered, you want to attach copies of those documents to support your changes.
If you go here Instructions for 1040X and read under the Assembling Your Return section on the bottom of Page 13, it will list everything you need to send along with Form 1040X. There should be a Filing Instructions page that should print with details on what else to include with that 1040x return. And you may want to consider sending it certified mail so that you know when they receive it. And be sure to sign it.