Now we get to the nitty-gritty of the form itself. First, you must obtain Form 1040X by printing it from IRS.gov or calling the IRS to request one. Unfortunately, for all those who love the convenience of e-file for filing a tax return, this particular form must be filled out on paper. (Most tax preparation services can also provide the form and help you fill it out.) You'll also need the "Instructions for Form 1040X," which is available at IRS.gov, and a copy of your original return and any supporting documents.
Examine the form itself for a few minutes, but don't fill out anything until you've had a chance to carefully read the instructions document. The instructions direct everyone who submits the form to fill out the top, including the tax year you are amending, your name, Social Security number, address, phone number and your filing status. Below that, you'll notice a lot of items that might or might not apply to you. The instructions include a chart to help you determine which lines to fill out and which lines you can skip. On the back of the form, the instructions direct everyone to include an explanation of the changes, a signature and date.
The IRS suggests using a hard copy of the original return you're amending and marking the changes in the margin. When you're done marking the changes, you can start writing those same amounts in the form. In Column A, write the amounts that were in the original return. In Column C, you'll put the new, correct amounts. Column B is simply the difference between the two. Write negative numbers in parentheses. For example, when Column C is less than Column A, write the difference is in parentheses in Column B. For items that don't change, write the same amount in Column A and Column C.
Be careful how one change might affect the amounts in other lines. For instance, if you change your adjusted gross income (AGI), which is your total income minus certain deductions, this could affect certain deductions that have a limit based on AGI, such as the allowable charitable contributions deduction or retirement savings contributions credit. Check with a tax practitioner if you have any questions.