Well, first and foremost, you have to develop a voice.
It can't really be said that you have a voice until you've learned the conventions and are in a position to put your own spin on "the rules". If you want to try sitting down and inventing your own rules from the start, good luck, but odds are, that "natural" voice is the voice of every rejected peice of slush.
Why?
Because the natural tendency is to write the way that is easiest for the author:
*long passages of expository prose, info-dumps a mile a minute
*inability to commit to a point of view
*shying away from real action in favor of distanced, post-action summaries
*cheep dialogue to advance the plot
*shallow characterizations
*poorly constructed descriptions of scenery and action
*cliche's everywhere
*passive sentence structure
*overuse of adverbs and adjectives
*overuse of pronouns
*overuse of thesaurus terms which nobody without a thesaurus can decrypt
*lack of attention to pacing
*predictable plotline
Any natural instinct will lead you to into all those "sins" because they require less work and less thought, and on some level, they are all instinctive. Not caring about the above makes life easy for you as a writer, but makes things miserable for the reader.
The trick is to learn and understand the "rules" and then put your personality back into the equation, bending and breaking said "rules" with a decent amount of authority. This is how the "voice" is developed.
If it seems ridiculous, then compare it to music. Even for a gifted singer, a certain amount of training is required before they find their voice. Things like "pitch" and "dynamic" are critical. Nobody likes an out of tune singer, or a screamer who has to strangle him/herself every time they hit a high note. It's just grating.
You may have mastered other forms of writing. But fiction - and especially novel length fiction - is a new ballgame. You have to come to terms with the "rules" before you can begin any discussion about your artistic integrity.