Go to addons.mozilla.org. If you want to install into Firefox, Flock, Mozilla Suite, or Songbird, click on "Install now". If you want to install into Thunderbird, follow the instructions on that page.
K-Meleon users may also be able to install Stylish, see this forum post.
The Stylish manage window lists all the styles you have installed. If you just started using Stylish, this window will be empty. From this window, you can add, edit, delete, enable, and disable your styles.
The Stylish manage window can be accessed in multiple ways:
Information on installing styles from this site.
To install styles from another site, click on the
in the status bar (lower right), click Write Style... -> Blank Style..., and copy and paste the code into the dialog.
Long story short, to be able to override styles on a web page, Stylish styles have to be more important than them. This can be achieved by adding "!important" after every value. For example, color: blue !important; rather than color: blue.
By default, styles are stored in a file called stylish.rdf in the profile folder. You can change this setting with the about:config preference "extensions.stylish.fileURL".