An author’s writing voice is much like a fingerprint, or even their actual speaking voice. It’s special, unique, and identifies them as an individual. When I pick up a book and start reading, the author’s writing voice identifies them, much like if they had called me on the phone. 
When I was in my teens and learning how to write fiction, I was also trying to find my own writing voice. The thing is, I found myself copying the writers I admired at the time – Tolkien, King, Silverstein, Bradbury, Asimov, to name a few. In fact, I would literally sit down with a copy of one of their books, along with a pad of paper and a pencil, and copy the text from the book to the page. Why? Because I was trying to BE them. I wanted to write like the people I admired and I figured if I copied enough of their work I’d somehow absorb their voices.
Alas, that wasn’t the case.
It did, however, teach me a lot about writing. I learned about flow, sentence structure, how narrative ideas tied together. And yes, I think it did eventually help me to find my own writing voice.
The thing is, your writing voice isn’t the one you start out with. It’s like learning to speak. You hear the people around you speak and you pick up their vocalizations, the way they enunciate, the regional dialect. Also, the speaking voice you start out with changes as you mature and the more you use it. When I was a child I had a speech impediment and had a higher tone. As I matured and went to speech therapy, my voice changed. It deepened (thanks, puberty!), I found more confidence with it, and eventually it developed into what I have today. 
My writing voice went through a similar evolution. I started out mimicking those that I admired and who inspired me. When I re-read some of the stuff I wrote back then I think the writing is immature. There’s no consistency to the narrative voice. Sometimes it reads like a cheap Stephen King knockoff or a bad Tolkien impersonator. Occasionally, there are glimpses of what would come later, but mostly it was bad imitation.
Unfortunately, there’s no quick and easy way to develop your writing voice. For me, I wrote. A lot. And then I wrote more. The one thing I can suggest is to not pay attention to it when you’re writing. You’ll become self-conscious and the writing will suffer. You’ll try to force it and that never works.
Just write. It doesn’t hurt to try the practice I mentioned above, copying passages or pages from writers you admire. If you’re a young or inexperienced writer, I recommend it. Don’t stick with just one writer, do this with several of them so you can get a better feel for the uniqueness of their styles. By doing this you can also learn things about how these authors constructed their stories. It’s like looking at a blueprint to see how a building was put together. It gives you a different perspective.
And most importantly, be patient. Some writers can develop their own style quickly, while with others it may take a while. Once you get there, though, you’ll know it. Also, remember that your writing voice will continue to evolve as time passes. Think of it as a fine bottle of wine that just gets better as we age.
RB