WordPress.org

1. Upload to your plugins folder, usually wp-content/plugins/.
2. Activate the plugin on the plugin screen.

All right, I installed it. Now what?

Load up the "Gatekeeper" management page under "Manage" and edit the default challenge, add some of your own, or both. The system will randomly pick a challenge from the list shown on the management page. If there's only one challenge, then it will always be used. If you manage to delete all of the challenges, then comment posting will be impossible. If that happens, you can use the "reset to default" link in the "Challenges" area of the management page to quickly re-enable comments.

You can also set a new master key, or generate a new random one, if you so desire. There's no particular reason to do this, but what the heck, the option is there. My favorite manually-set master key is "spammerssuck" but you may feel otherwise.