Very carefully! This document—which your employer will ask you to fill out when you start a new job—indicates to the company how much it should take out per paycheck for income taxes. Ideally, you don’t want to have too much of your paycheck withheld (since you’d basically be giving Uncle Sam an interest-free loan) or too little withheld (since that means you may owe come April 15).
Getting this jusssst right isn’t as easy, however. The amount earmarked for taxes is generally determined by the number of “allowances” you select—you get one for yourself, another for your spouse, and one for each of your kids, for example. The more you have, the less tax will be withheld, since a certain amount of income is exempt from taxes based on these situations. What the form doesn’t take into account are the specifics of your finances outside of your job for the current year—whether you have significant income from freelance work, say, or a large amount of capital gains or a big pot of deductible expenses. You can use the IRS Withholding Calculator to help you determine the correct withholding, but still don’t expect to hit the nail exactly on the head.
The good news is that your decisions aren’t permanent. You can adjust your W-4 at any time, and you might want to if you find that next tax day you owe a considerable amount or are getting a fat refund. You should also revising your withholding if you get married, have a baby or buy a house as these changes may change your tax situation.