There are 5 steps. First you have to enter what filing status will you use on your 2012 Income Tax Return:
Single, Married filing joint return, Married filing separate return, Head of Household or Qualifying widow(er).
Click here to enter 2012 Withholding Calculator
You will also have to enter info about Child and Dependent Care Credit and Child Tax Credit. In Income and Withholding section you will have to enter he total wages, salary, and tips (excluding bonuses) you expect to receive in 2012 (Enter military retirement or taxable pensions as separate jobs.), total 2012 contribution to a tax-deferred retirement plan, FSA or HSA, total Federal income tax withheld to date in 2012 (including amounts withheld from bonuses or which you expect to have withheld for bonuses), Federal income tax withheld from your last salary payment etc.