How do I file an injured spouse form if I already filed my taxes for 2016

The IRS allows you to choose and file Injured Spouse with the amendment or by itself, after you have already filed. 

To file by itself NOT using TurboTax, you can use below link to print and mail form 8379:

https://www.irs.gov/pub/irs-pdf/f8379.pdf

https://www.irs.gov/pub/irs-pdf/i8379.pdf

To generate Injured Spouse in TurboTax at this point -since you already filed your tax return - you have to amend your tax return. However, when you are done with the amendment, you can still choose: print amendment and injured spouse and send it together; OR only print Injured Spouse form 8379 and mail it by itself.    Here are amending instructions.

Note:  If you file the form by itself and you filed your original joint return electronically, you must mail the form to the IRS center for the area where you live (select the link and select your state for addresses). 

https://www.irs.gov/uac/Contact-Your-Local-IRS-Office-1

Here's TurboTax data entry:

1. Federal Taxes
2. Other Tax Situations
3. Scroll down to Other Tax Forms, select Miscellaneous tax forms
4. Select Report an injured or innocent spouse claim

NOTE: In TurboTax, when you are filing Injured Spouse Form only (F8379), please do no answer Innocent Spouse questions, simply bypass them. 

You must, however, answer "Tell Us More about Your situation"  questions and check any boxes that apply to you. The selection in this section determines if you qualify to file Injured Spouse.  While you will not see the Form itself, your answers will be automatically populated on F8379.