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1. Upload the contents of the zip archive to your plugins folder, usually wp-content/plugins/, keeping the directory structure intact (i.e.wp-monalisa.phpshould end up inwp-content/plugins/wp-monalisa/`).
2. Activate the plugin on the plugin screen.
3. Visit the configuration page (wp-Monalisa) to configure the plugin (do not forget to check the comment forms id)
4. Optional If you would like to change the style, just edit wp-monalisa.css

Update: If you update from a former version please do not forget to deactivate and actiate the plugin once, since database changes will only take effect on activation. You settings will not be deleted during deactivation.