Use lines 3, 4, and 5 below to have additional withholding per pay period under special agreement with your employer. A W4 form, or "Employee's Withholding Certificate," is an IRS tax document that employees fill out and submit to their employers.To ask your employer to withhold an additional amount each pay period, complete lines 3, 4, and 5 on Form IT-2104, as applicable. You will need to fill out a W-4 form when you start. This video goes over how to fill out a 2021 Form W-4 if you are single and the basics behind Form W-4. Complete Form W-4 so that your employer can withhold the correct federal income tax from your pay. Cut in half for each job to calculate withholding. Use an online estimator to determine a specific amount to have your employer withhold each pay period. The W-2 form reports an employee's annual wages and the amount of taxes withheld from his or her paycheck. (Your withholding will be most accurate if you complete Steps 3–4(b) on the Form W-4 for the highest paying job.) Step 3: Claim.