Cover letters: Using a Microsoft Word template vs making your own

For a class this last spring, we had to make cover letters as homework. I used one that I had already made (especially because it had already gotten me an interview) but I got points marked off because I had just used a Microsoft Word template, rather than doing it his way -- typing it up on a blank word document in a special format.

Now I'm trying to apply for a job and I'm starting to second guess myself. So long as I'm writing a letter that conveys my interest in the job and uses proper spelling and grammar -- is there a wrong way to do a cover letter?

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