WordPress.org

For WordPress:

1. Extract .zip file to the /wp-content/plugins/ directory
2. Activate the plugin through the 'Plugins' menu in WordPress

For WordPress Mu:

1. Extract .zip in the /wp-content/mu-plugins/ directory
2. That's all.

Or:

1. Extract .zip in the /wp-content/plugins/ directory
2. Wait for your bloggers to choose whether activate it or not.