Please see the below information obtained from IRS.gov regarding how to file Form 8379. I have also included the IRS link for your reference.
How To File
You can file Form 8379 with your joint tax
return or amended joint tax return (Form
1040X), or you can file it afterwards by itself.
File Form 8379 with Form 1040X only if you
are amending your original return to claim a
joint refund.
If you file Form 8379 with your joint return,
attach it to your return in the order of the attachment sequence number (located in the
upper right corner of the tax form). Enter
“Injured Spouse” in the upper left corner of
page 1 of the joint return.
If you file Form 8379 separately, please
be sure to attach a copy of all Forms W-2
and W-2G for both spouses, and any Forms
1099 showing federal income tax
withholding, to Form 8379. The processing
of Form 8379 may be delayed if these forms
are not attached, if the form is incomplete
when filed, or if you attach a copy of your
joint tax return.
https://www.irs.gov/pub/irs-pdf/i8379.pdf
