The principles listed here are sound and helpful. If you follow them, your writing style will undoubtedly improve.
But do not adhere to them as rigid rules or you will suffer in your ability to express yourself. Passive voice exists for a reason. Long words can add variety, rhythm, and color to your prose. Elongated sentences can help give your writing a flow that a mere parade of short sentences can never hope to achieve, not even after a thousand rewrites. Or not. All such items can be misused as well, and the books are replete with bloated forms of expression used by lawyers, politicians, educators, administrators, and the like who would not know a simple word or sentence even if it stood before them doing somersaults. The key is to know sound principles for clear and concise writing and then to apply them with a rhythmic ear for balance in your forms of expression. That means, yes, use passive voice, long words, and flowing sentences as needed to add grace to your prose but always with the baseline in mind: that is, to communicate in ways that are clear and concise and that people will readily understand for your stated purpose (formal style for formal settings, casual for casual, and whatever fits for anything in between).
I note all this because, years ago, I consciously and diligently set about to attempt to master writing and stumbled upon the rock of "simplicity" during such stretches in my learning process where I had assumed that all one could do was follow such rules. Any attempt to apply such rules one-dimensionally is a mistake, and you will regret trying it. Follow sound principles, by all means, but not dogmatically.
The other major keys to good writing are depth of language skills and extensive reading. No one will read your work unless you have something helpful to say. You get this by working hard to develop your skills, and lots of writing (and reading) is vital to this process.