Tax is computed at the standard rate of twenty percent on your gross pay up to the amount of your weekly standard rate cut-off point. Any balance of gross pay that is more than the standard rate cut-off point is taxed at the higher tax rate of forty one percent. The tax that is computed at twenty percent is summed up with the tax computed at forty one percent to arrive at your gross tax figure. This figure is then reduced by the amount of tax credits to get to your tax payable in that week.
You can go to any IRS office near you and they have papers on how to take out taxes on payroll checks.They can give all things needed to help yu set up how and where to send the with holdings.Good luck
Go to IRS.gov, they have a publication called Circular E that contains the tables used to compute the Federal withholding on employees wages. It is broken down according to filing status (Married or Single) and exemption amounts that the employee has designated on their W4. Also it depends on how often they are paid. It is a complicated formula but with the tables it is easy to see exactly the correct amount to withhold.