If you are mailing your Illinois tax return, you should include copies of any 1099 with Illinois state withholding reported on it.
- Attach a completed Schedule IL-WIT and all withholding forms (W-2s, 1099s, etc.) to support the amount you claim as Illinois Income Tax withheld on Line 25. Form IL-1040 instructions. See Tips to Speed Up Processing on page 2
TurboTax will generate and print a Filing Instructions page which will list what you need to include along with the mailing address.