While it may seem simple, finding your voice is one of the trickiest elements of craft to master. I blame the way we are taught to write. In school, we read text books and are expected to regurgitate what we've just read in the form of essays and reports and form letters. (Don't get me started on book reports...)
The assignments which received the best marks were usually dry and rote. Any personalization was crossed through in red pen and excised as irrelevant. For young creatives, this is discouraging and defeating. When we were told to write "stories," our teachers tended to focus on the form and structure of our narratives instead of the stories themselves. Commas flying all over the place. Run-on sentences bifurcated and trifurcated (Is that a word?) by periods. Oh the frustration created by those pesky semicolons! Alas!
That said, as writers, each of us needs to consider our voice when we write. According to Literary Devices, "A voice in literature is the form or a format through which narrators tell their stories. It is prominent when a writer places himself herself into words, and provides a sense that the character is real person, conveying a specific message the writer intends to convey. In simple words, it is an author’s individual writing style or point of view." But what does that mean, exactly?
If you are a conversational writer, trying to write a technical article would feel disingenuous and false. Readers would pick up on the insincerity. Likewise, a romance writer delving into high fantasy may come across as trying too hard. Write the way YOU write, not the way you think you should. If your story is strong, readers will appreciate it.
But, as with any craft element, voice has nuances and variants. Author voice may differ from character voice in fiction. The narrative surrounding characters, their actions and their world will reflect author voice - unless the author is not the narrator, i.e. another character is telling the story. Character voice should remain true to the character, not the author or another narrator. If the author is from the modern-day United States, while the narrator is a Spanish Conquistador relaying the actions of the indigenous tribes he found in the New World, it would be strange and off-putting for the narrator to say "The natives shouted 'Geronimo!' as they jumped into the creek." The narrator is unreliable in this case.
Instead, this narrator, the Conquistador, would more likely say, "En efecto, the natives plunged into the ravine, while brandishing their spears and screaming unintelligible battle cries!" Readers understand the loose cognate of "in effect" with the inclusion of the Spanish term, thus lending the narrator credibility. The phrasing supports the time period and attitudes of both the Conquistador and the native peoples. See, voice matters.
So, the upshot. Write the way you write and stay true to your characters. Easy enough.