You may complete a new Form IL-W-4 to update your exemption amounts and increase your. Illinois withholding.El Formulario 1099 consta de una serie de documentos a los que el IRS (Servicio de Impuestos Internos) se refiere como "declaraciĂłn informativas". You must file Form IL-1040-X, Amended Individual. Income Tax Return, if. I am going to show you how to fill out sales and use tax form st1 for illinois sales tax.