Writing scripture is not all that difficult. In fact, the less sane you are the better you will be at it. Be sure to eschew any honesty, clarity, and fairness. You will not need them here. Rather, you'll want to be as creative, eloquent, evasive, and cryptic as you can. You need not worry about contradictions, for future followers will easily gloss over them and then pride themselves for their mystical understanding and faith.
Be sure to stow yourself away. The environment must be challenging in that there must be few comforts and please be sure to minimize food intake. You'll need water, though polluted serves just fine. Mind-altering substances are not required, yet they do provide some bizarre effects. Just read Ezekiel or The Revelation of John.
Next, you'll want to evoke from within you the most intense anger and hatred. Inspire within you the most caustic vitriol to fuel the diatribes and long homilies. Don't worry about fatigue, for then you'll write the more boring passages that are oftentimes skipped over anyway.
Then write. Write like the wind. Write furiously and without rest. Write quickly and be sure to scribble incoherently at times, for later generations of followers will fight to the death over competing interpretations of a missing (or extra) dot here and a incomprehensible word there. You need not worry, for you're inspired! Just be sure to be as eloquent as you can when you're spewing the hate.
Once you finish writing, then you'll sit back and be amazed at your work. The result? Another Isaiah, Qur'an, Tanakh, New Testament, Pali Canon, Books of Enoch (There's at least three of them folks), or Book of Mormon. You can't get any worse than these.
One final note: it is a colossal waste of time. The world's populace is enslaved be those that impose the tyrannical guidelines of the various scriptures upon them. Writing scripture is an unhealthy, hateful, and unethical act. I wouldn't recommend the activity for anyone: past, present, or future.