If you've already filed and your return has been accepted, you will have to mail the form to the IRS using TurboTax to create the form.
If you are using TurboTax Online, sign in and make sure you have selected Tax Home. Select Show next to Your tax returns & documents, choose the year you want to file the form for, then select Amend (change) return.
If you are using the CD/Download product, open your return.
When your return is open, In the search box, search for injured spouse and then click the "Jump to" link in the search results. Answer Yes to Do you want to claim innocent or injured spouse relief? and carefully follow the onscreen instructions for Injured Spouse.