You should send tax returns and their related forms and tax payments, if any, in separate envelopes to the address indicated in the instructions for the form. Do not send state income tax returns with your federal tax returns! Do not combine tax payments for more than one tax year in one check. The IRS will apply the payment it to only one year. They will send you a refund for the excess payment made on the first year and a bill for the taxes due on the second year.