"Voice" is a confusing concept in fiction because sometimes we use it to refer to a writer ("Hemingway's distinctive voice") and sometimes to one of a writer's characters (the barber in Ring Lardner's classic "Haircut"), which means one writer can have, for different characters, different voices. Unless he doesn't.
I prefer to think of "voice" as belonging to a specific character in a specific story, and if you're writing in first person, it's critical. In fact, to me it's the only reason to write in first person, since third is more flexible in point of view and in distance, allowing the author to both go into the character's mind AND give some decription of him and his behavior from the outside. It also feels less contrived, since a perennial first-person problem is the question: Why is this person telling me this, and in such perfect rhythm and pacing? It doesn't feel real. On the other hand, first person lets you capture directly the patterns of thought, focus of attention, and nuances of the narrator's mind.
If I haven't got the right voice for first person, I can't even begin to write, since voice is evident from the first paragraph. I think I've found the voice for my YA character, Susan Coleman. It took some noodling around, and I'll have to go back and rwrite some of the earlier noodling because now it doesn't fit, but that's SOP for me. I like her voice.
Now all I need is the rest of my plot, which is emerging slowly as I write. That's how I do it. There are more efficient methods, but this is the only one I've ever found that works for me.