Although it's common to take a closer look at your income tax at the start of the new year, that's not the only time you can make changes to your income tax selections.  You can adjust your W-4 at any time during the year.   Just remember, adjustments made later in the year will have less impact on your taxes for that year. 

It's important to remember that if your circumstances change—for instance, if you get married, have a child, or start a second job—you can adjust your withholding as needed.

The purpose of these forms is so your employer can withhold the correct Federal & State income tax from your pay.

• If too little is withheld, you may generally owe tax when you file your tax return and could potentially owe a penalty.
• If too much is withheld, you may generally be due a refund.

This section asks for employees for their name, address and Social Security number.  Also - how do you want income tax calculated and withheld from your pay check?  (single, married filing jointly, etc.)

This step is the most significant change from previous W-4s, and it calls for employees to account for money earned from other jobs, as well as a spouse’s salary. This step will enable an employee to get the most accurate amount to withhold, and there are several options to take.
Employees should only do one of the options, which include using the IRS Withholding Estimator (most accurate) or the Multiple Jobs Worksheet (roughly accurate). Self-employed individuals also should use the estimator

https://www.irs.gov/individuals/tax-withholding-estimator
The employees who would complete this step include those who hold more than one job at a time or are married and filing jointly (and the spouse works). The filer would then complete steps 3 to 4(b), and to get the most accurate withholding it should be done for the highest-paying job.

Can Employees Skip Any Steps?

Employees who have one job and are not married or are married and filing separately can skip step 2. Step 4 is optional (more details below).

These are tax credits, not allowances. The income levels changed as did the credits, and individuals now phase out of the tax credit at a higher income threshold. A filer with an income level of $200,000 or less ($400,000 or less if married filing jointly) will receive a $2,000 child tax credit for any dependent younger than 17, as well as $500 for any other dependent, including an older parent being cared for in the household.

This step includes listing other sources of income that are not job related such as interest, dividends, and retirement income, as well as other deductions other than the standard deduction. Employees might have questions about this section because it allows visibility into their entire income amount.
This step is optional. It would give employees the most accurate withholding amount but it requires a lot of work to fill out the Deductions Worksheet.
Employees have the option in this section – 4(c) – to enter any extra withholding or additional tax they want taken out each pay period.

How Can an Employee Claim Exemption from Federal Withholding?

If an employee meets certain conditions (under General Instructions, Purpose of Form, Exemption from withholding), they may claim exemption.  An employee who qualifies for exemption, must only complete steps 1(a), 1(b), which is their name, address, and Social Security number, and step 5 (signature).

They must also write the word “Exempt” beneath Section 4(c) and above the line that begins step 5. It should be noted that these employees will need to submit a new Form W-4 by Feb. 16, of the following calendar year in order to continue claiming exemption.

Incomplete, unsigned and/or illegible forms will not be processed.   Until a completed form is submitted to the Payroll department, the income tax will be withheld from your wages as if you are single with zero allowances.

Please consult your tax professional if you have additional questions that have not been answered here.  There are many online assistance resources on the IRS.gov website (noted above).  You may also visit: ftb.ca.gov for assistance with the CA Tax Withholding Form.   The Payroll department is unable to provide tax advice.