The proper way to file Form 8606 is attached to Form 1040-X.
Form 8606 can be mailed by itself only when you are otherwise not required to file a tax return.
Did you do a true backdoor contribution that is tax-free, or planned to be?
in that case Form 8606 will show a taxable amount of zero.
Do you have the correct taxable amount on Form 1040 Line 4b and did not take a deduction? (Schedule 1 Line 20)?
If that is all true then only Form 8606 has to be attached.
After you e-File,
get Form 1040-X from IRS website and mail it in with your <year> 8606, which you can also get in fillable PDF under "prior forms"..
Note: when you are not changing any dollar amounts on your amending 1040-X, you can leave all the lines 1-23 EMPTY.
Part II explanation: "didn't include Form 8606 with e-File".
Since e-File is closed, you will have to mail it.
Done this way, you sign 1040-X, not 8606.
Do not include your old 1040 nor your revised 1040 because they are identical.
If you took a deduction and now want to elect non-deductible,
your 1040-X dollar amounts will not be zero, Schedule 1 Line 20 will be changed to -0-.
Still, do not include your old 1040 nor your revised 1040 because the Form 1040-X reflects any changes there and becomes your new tax return.
@SmallBusinessSurvivor
