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I recommend that you begin sending them in with the very next payment which is due September 15th. This payment should, if possible, include the first two payment amounts as well (in other words the equivalent of the first three payments combined). Then follow up by sending in the next payment which is due January 15th.
If you do this you may be subject to a late payment fee for the first two payments (determined when your returns are actually filed) but the amount will be less than if you do otherwise.
To calculate the payment amounts simply divide last years tax by 4. This is the amount which would be due with each quarterly payment. On or before September 15th you should send in 3 times this payment and then 1 times the payment in January as I stated above. I also recommend rounding the payment up to the next $5 not only for simple record keeping but to build a protection for year end which is not really that far away. Finally, prepare and submit your returns as early as possible to help reduce any potential penalties. It has the added benefit of getting your refund sooner if that happens.