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Upload to your plugins folder, usually wp-content/plugins/, keeping the directory structure intact (e.g. wp-greet.php should end up in wp-content/plugins/wp-greet/).

Activate the plugin on the plugin screen.

Visit the configuration page (Options -> wp-greet) to configure the plugin (do not forget to add the forms page id)

Optional: If you would like to change the style, just edit wp-greet.css