The deadline for submitting any estimated tax payments for the year 2009 is Friday, January 15, 2010. The actual mechanics of sending in estimated payments are pretty easy. Either mail in a check to the Internal Revenue Service, or submit an online payment using the Electronic Federal Tax Payments Web site. Actually figuring out how much to send in can be a bit more complex. For most people, the simplest answer is to pay in at least 100% of their 2008 taxes, minus any withholding or other estimated payments. This will ensure that the IRS does not charge you an interest penalty for underpaying. Any additional balance due can then be paid by April 15th, 2010, using the extension voucher.
If you are mailing in your payment, make sure to enclose a 1040-ES payment voucher. This has all the information the IRS will need to process your payment correctly. On your check, in the memo field write in your Social Security Number, the tax year (2009), and "Form 1040-ES". Make your check payable to the "United States Treasury." Do not abbreviate ("US Treasury") or write in "IRS" or "Internal Revenue Service"). And, finally, make sure you mail it to the right address. As long as the payment is postmarked by January 15th, you'll be fine. Make a copy of your check before mailing it off, or keep a copy of the canceled check (if your bank still provides you with this) along with your other records for 2009.
If you are making a payment through EFTPS, here's a couple of tips. First, make sure your EFTPS account is set up for the checking account you want to use. In my case, I needed to change the bank account information. This required setting up a new EFTPS login with its own PIN and Password. This will take you an extra 15 to 20 minutes or so. Next, make sure your indicate the correct tax year for the payment. This will be for tax year 2009. I made the mistake of typing in 2008 (mostly because I still think 2008 is "last year"). Review all the details in the confirmation page before submitting your payment. Print out the confirmation receipt and keep it with your other tax records for 2009.