For general help with Stylish, see the help docs on userstyles.org.
Writing a user style from scratch requires a working knowledge of CSS. There are many CSS tutorials online, most of which are better than anything I could write. For those have limited knowledge of CSS, my recommendation would be to take an existing style that does something similar to what you want, and attempt to understand and modify it to suit your needs.
For those with some knowledge of CSS, here are some articles specifically about writing CSS for user styles: