The easiest way for any writer to develop their true voice is to write as naturally as possible, concentrating on story instead of on word choices or sentence formation.
The true measure of voice in writing is how close words on paper come to transferring the narrative flow inside the author’s mind to the mind of the reader.
Creative writing students are often required to write a few paragraphs imitating the voice of a writer known to have an easily recognizable style. It’s a useful exercise for discovering your own.
Every writer is born with a voice. The trick is in separating it from all the others absorbed during years of reading, and then using it until letting it flow becomes second nature.