Mail the 1040X with any documents related to the changes you made. For example, if you just received another W-2 or 1099 after you filed your tax return, then you would need to attach those forms (to the front of the 1040X).
If you owe tax, enclose (do not attach) your check or money order in the envelope with your amended return.
Do not attach a copy of your original return, correspondence, or other items unless required to do so.