Calculate Your Projected Taxes
The first step is to decide how much, if any, estimated tax you need to pay.
Estimated taxes depend on your total taxable income and overall taxes. An easy estimating method is to use your last tax return as a base point. For 2011 estimated taxes, look at each line item on the 2010 return. Note if the 2011 amount will be higher, lower, or about the same as 2010. Update the number of dependents claimed, if needed. Determine if you will still be able to claim any credits claimed on the 2010 return, or if you can claim new 2011 credits. Note any new type of income that you are expecting, such as self-employment income, rental income, or a withdrawal from an IRA or pension plan.
Now get a blank tax return (or, even better, use a tax software program) and fill it in with these updated numbers. Calculate this projected tax return, using 2011 rates and deductions, if available. Otherwise, use 2010 rates and deductions - you should still have a good rough projection of 2011 taxes. If you have self-employment income, don't forget the self-employment tax.
The Form 1040-ES also has a worksheet you can use to figure the amount if you prefer.