I think a writing voice is a lot like an accent: you can't hear your own and don't think you have one. Others can recognize your writing voice, but if you ask them to describe it, they may have trouble putting their fingers on what makes it unique. It's hard to pin down and can't truly be taught in a class, but it is something every serious writer needs to develop in order to stand out and be successful.
My first serious attempt at finding my own writing voice was to steal someone the voice of someone else. I was (and still am) a huge fan of Douglas Adams, with his inimitable British style of run-on sentences and weird analogies, and his complicated relationship with grammar. For example, he wrote this gem in Life, the Universe and Everything: “The party and the Krikkit warship looked, in their writhings, a little like two ducks, one of which is trying to make a third duck inside the second duck, whilst the second duck is trying very hard to explain that it doesn't feel ready for a third duck right now, is uncertain that it would want any putative third duck anyway, and certainly not whilst it, the second duck, was busy flying.”
Perhaps Douglas Adams' greatest strength, which I failed to appreciate in my naive youth, was his ability to couch profound messages about life and scathing critiques of Western civilization and/or the human race in such a humorous way that it never felt the slightest bit preachy or pretentious. I was in seventh or eighth grade when I attempted to copy him. But I'm not British, so of course the results were sub-par and are painful to read today. But I'd like to think that a piece of his style stayed with me and that a trace of his voice can still be glimpsed in mine without conscious effort on my part. On a slightly different note, I would also recommend his non-fiction work Last Chance to See which is every bit as hilarious and even more thoughtful than his better known Hitchhiker's Guide to the Galaxy series.
A lesser-known author that I’ve tried to imitate was Campbell Black. I read his novelization of Raiders of the Lost Ark, and fell in love with his prose. At that time the Indiana Jones franchise was a blank slate and Campbell Black made quite an effort to turn it into serious literature. One aspect of his style was stringing adjectives or sentences together without “and.” For example, “The jungle was dark, verdant, menacing.” Over the course of the book, this created a rushing, breakneck speed that I wanted to replicate. He was also talented at getting into characters heads by seamlessly mixing their thoughts with description in second person. I gave up my imitation of Campbell Black more quickly than my imitation of Adams, but again, I hope I took some of it with me.
I've also channeled Dr. Seuss a couple times. The less said about that, the better.
My writing voice is heavily influenced by the fact that I’m on the autism spectrum. My autism gives me a different perspective on things than most people and a naturally different way of articulating myself. A couple years ago one of my professors, Russ Beck, described my voice as “almost completely stripped of metaphor” and “just unique enough that I'm nearly willing to read anything you're writing.” Of course, my autism has made writing more difficult for me in some ways as well.. I had the damnedest time writing dialogue that came anywhere close to the way real people talk. And as hinted, figurative language doesn't come naturally to me, with the exception of cliches, and even some of those pissed me off when I was little. I've tried to get better at metaphor since many writing classes require it.
When you're comfortable with your writing voice, you can take risks and play around with it. You can try breaking rules. Good authors break rules all the time, but they know when and how to do it so their work is clever instead of godawful. I like to compare it to traffic laws. Most traffic laws of exist for good reasons, but police officers are allowed to break them. However, police officers are required to have both special training and adequate justification to do so. If they just ran red lights or drove in the wrong lane whenever they felt like it, there would be repercussions.
I think imitating authors is a good way to begin discovering or creating your writing voice. Even if your initial results are as terrible as mine were. But I think the most important thing you can do is read authors you admire and let their strengths seep into your writing (and of course, you can’t go wrong by writing consistently). Like learning to talk, writing takes practice and guarantees plenty of mistakes. There's no specific set of foolproof steps that I, or anyone else, can give you to develop your writing voice. You may not notice your voice developing, but eventually your readers will.