The 1 sentence you should use to start your cover letter, according to ZipRecruiter's CEO

It takes the average person six weeks to apply for and land a new job. Throughout the ups and downs of the typical search, there's one part of the process is usually more daunting than the rest: writing a cover letter with each application.

The challenge of writing a good cover letter may be why six out of 10 job applicants don't submit one at all, according to a survey from resume advice site ResumeLab.

But HR professionals still say these documents are crucial to their decision-making process, and they overwhelmingly give preference to candidates who include one with their application. The ResumeLab survey of 200 hiring managers also found that, 83% of the time, a great cover letter was enough to sway them to extend a job interview to a candidate with a less-than-perfect resume.

Ian Siegel, CEO of jobs marketplace ZipRecruiter, understands why job seekers don't know the best approach to writing a good cover letter.

"I think there's a traditional best practice way of writing a cover letter that's filled with bad advice for the current generation we're in," he tells CNBC Make It. "It's more of a demonstration of communication competency and attempt to provide a short summary of your skills and experience."

Using a cover letter to detail your qualifications is a big mistake, Siegel says — a resume should already accomplish this. In fact, the biggest mistake a job applicant can make is by starting their cover letter by talking about themselves.

Instead, applicants should begin by talking about the company, and specifically why they're excited by the prospect of joining.

"Don't talk about yourself first; talk about them first. My golden rule is to make them feel excited," Siegel says, noting that hiring managers spend just 20 seconds reviewing every application, on average. "Your excitement will get them excited — it's the best way to get their attention," he adds.

He recommends job seekers open their cover letters with the following sentence: "I'm so excited to apply to this job because ..." and fill in the blank about the company's product, service or mission.

Beyond that, Siegel boils down the most important things to include in a cover letter to three points.

"Show enthusiasm, show you've done research, and show you want to come in there and make a contribution," he says.

Following this four-step formula can help job seekers better understand which jobs are most appealing to them, and therefore which ones are more worth their time to apply.

The good news for the cover letter-averse is that, as more companies allow people to apply to jobs via a mobile device, they're less likely to require the additional document. In such cases, they may instead opt for requiring what Siegel refers to as a cover note, where applicants must type out their elevator pitch in a few sentences.

In such cases, the objective is usually much clearer: Explain why you applied (not why you're qualified).

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