Don't spend 500 words describing your writing style.
Give it 3 examples of things you actually wrote.
Then say:
"Study these examples. Don't copy their wording. Extract the patterns in sentence length, vocabulary, rhythm, humor and structure. Use those patterns in anything you write for me."
This is called style induction.
The model figures out the style from examples instead of relying on your description.
And honestly, this is one of those cases where: show, don't tell works ridiculously well with LLMs.
Your own writing is usually a better style guide than:
"Make it casual, professional, engaging and human."