This was basically a way for residents of states without income tax to have an additional itemized deduction, since they have no state income tax to deduct. So it is not instead of itemizing, it is a line item on the itemized deductions form (schedule A) and the choice is between putting your state income tax or state sales tax on schedule A. If you live in a no state income tax state, then you would be using the state sales tax figure. For others, they need to determine if the state income tax or sales tax amount is higher. Of course there is still the overall decision whether to use the itemized deductions or the standard deduction, choosing whichever route gives you the greatest overall deduction.
As said previously, there are set amounts to deduct (a calculator can be found on the IRS site), unless you have the receipts to total up and enter a figure yourself. You can also add on for unusual, large purchases, such as cars, to the suggested sales tax amount as that original figure does not account for "big ticket " purchases since they typically do not occur every year.