You’re probably referring to Form W-4, “a particularly confounding, boring form that everyone has to fill out and no one understands,” said Russell Garofalo, founder of the tax firm Brass Taxes. But if you get it wrong and your employer withholds too little from your paycheck, you can be hit with a tax bill — and possibly a penalty.

This tax season, the form underwent a major overhaul to reflect the changes to the tax code. Before the redesign, it was largely based on the number of so-called “allowances” you could take. Allowances represented personal exemptions, or the amount of money you could deduct for yourself and dependents.

But personal exemptions are no longer available, so the new form walks filers through five steps that try to capture all income sources and also account for tax breaks.

Your best shot at getting it right if you do it yourself? Grab a caffeinated beverage, a recent pay stub and your latest tax return. Then go to the Internal Revenue Service’s tax withholding estimator tool online, which will provide instructions on how to fill out your W-4.

“What the I.R.S. has built is user-friendly, but it is kind of like doing your own tax return on their website,” said Julie Welch, an accountant in Leawood, Kan. “It is pretty precise.”

Because there isn’t much “fluff” built into the amount withheld, Ms. Welch said you need to be sure you are entering everything carefully. Don’t skip items like the bank interest you earned, for example, or you will risk withholding too little.

There’s also a line for any outside income you’re paid. But do you really want your employer knowing that? Probably not, which is why tax pros suggest using line 4(c), instead, which lets you withhold a flat extra amount from each paycheck, with slightly more privacy. (Or you can set money aside and make estimated quarterly tax payments on your own.)

The I.R.S. tool also has a neat refund slider — it’s fun, to the extent that taxes can be — so if you wanted no refund or a $500 refund, you can bake that into the results generated as well.

2. I owed the government money last year for the first time, and it appears that I do again. What happened?

A big gift of the 2017 tax legislation is also among its mysteries: What sort of work do you need to be engaged in to obtain every bit of a new 20 percent tax deduction on qualified business income?

One pertinent Internal Revenue Service explainer on the topiccomes with a bright red “Caution” warning, noting that it is undergoing revision. Whatever words spring forth, this area may well be an audit bonanza for the I.R.S.

Plenty of self-styled advisers and consultants now approach Minnie Lau, a San Francisco accountant, with mistaken notions of how they can get the tax break. Ms. Lau tells these clients that they may be doing themselves a disservice, because high-income consultants may face limits on the deduction. “The word consultant is the vaguest term in human history,” she said. “It tells me nothing. Consultant of what, dude?”

Quite often, her aim is to get people to label their work (accurately) as something other than consulting, so as to qualify for as much of the deduction as possible. “I’m asking a question to get you in a better place,” she said.