WordPress.org

For WordPress µ

1. Upload the simpler-css.php file to the /wp-content/mu-plugins/ directory. The other files should not be uploaded, and the file cannot be in a subdirectory.
2. You're done! As a mu-plugin, Simpler CSS is automatically enabled for all blogs.

For normal WordPress installations

1. Upload the simpler-css directory to the /wp-content/plugins/ directory.
2. Activate the plugin through the 'Plugins' menu in WordPress.