Almost every American citizen, resident, or legal nonresident alien is required to file an income tax return every year with the Internal Revenue Service. This is usually done by filing one of the different forms of the 1040 series available (1040, 1040A, 1040EZ, 1040NR, etc.).

However, the American tax system is extremely complex and it is easy to make mistakes when doing so. When a mistake is found, or you discover that there was a tax credit or deduction that you could have claimed but did not, you can amend your previously filed 1040 by submitting a form 1040X.

In general it can be safely assumed that any amendments to a previously filed tax return, and especially an amendment that requires the IRS to make an additional payout, will undergo more scrutiny than the original filing. Therefore it is absolutely essential that you be extremely careful when filing your amendment on form 1040X.

Mistakes can not only result in a lengthy delay and an audit, but if the mistake is deemed excessive by the IRS they can result in massive penalties.

The first step is to fill out a new 1040 form for the year that you intend to amend. Although you will not be sending this new 1040 form to the IRS, this can be extremely useful for ensuring that your 1040X amendment matches your original filing.

When filling out the new 1040 form with the changes, be sure that you are using the correct form for the year in question, as sometimes the form – or the acceptable range of information recorded on the form – changes from one tax year to another. So if you are filing a Form 1040X to amend your 2008 income tax return, be sure that you are using a blank 2008 1040 to make your amended version.

Once you have a copy of your original return as filed with the IRS and the new version taking into account the changes you want to make, you are ready to fill out the Form 1040X. You will notice that the form 1040X has three columns: Column A represents the information that you originally filed with the IRS, Column C represents the information that should be on your newly filled out 1040 form reflecting the changes you want to make, and Column B represents the difference between Columns A and C.

This also makes it easy to double check your numbers as any two columns, properly calculated, should equally the third column. So Column A plus Column B should equal the numbers in Column C and so on.