Withholding is typically treated as withheld ratably throughout the year. The withholding is divided by 4 and the result is entered in each of the columns. If you made estimated payments instead, these payments would be applied to the period in which they were made.
When you use the annualized method, you'll scroll down to the annualized income worksheet and enter the income earned in each quarter to determine what your required minimum payment would be based on the amount of income earned during that period.
