If IL is your home of record (entered the service from IL), the DFAS Form W-2 should show all of your income as IL income (Boxes 15-17).
If IL is not your home of record,
the DFAS Form W-2 should report all of your income to your state home of record (Boxes 15-17).
In either case, Enter the W-2 exactly as it was issued to you.
Answer all of the Personal Interview questions as if you lived in your state home of record all year, Your military earnings are considered to be from your state home of record.
If IL is your home of record, your military pay is exempt from IL tax. See page 3 of Publication 102
Illinois Filing Requirements for Military Personnel