The best way to fill out a tax return check is to first gather all the necessary documents and forms. This includes the tax return, any necessary supporting documents, and the check. Once these are gathered, the taxpayer should fill out their name, address, and social security number on the check. Then, the taxpayer should fill out the amount of the check. The taxpayer should make sure to write the total amount of the refund in words and numerals. Next, the taxpayer should attach the tax return, any documents, and the check to a return envelope and mail it to the Internal Revenue Service (IRS).