If you send in a payment by check, you must also complete Form 1040-V and include it with your tax return. The IRS requests that you don't staple the check to your return, but simply enclose it in your envelope.
The file is in Adobe Portable Document Format (.pdf), and you will need the free Acrobat Reader to view and print the files.
Form 1040-V, Payment Voucher
Federal Income Tax Obligations