1. Install the plugin from the WordPress plugin screen.
2. Activate the plugin.
3. Go to the plugin settings page and build the index following the instructions there.
4. That’s it!
Relevanssi uses the standard search form and doesn’t usually need any changes in the search results template.
If the search does not bring any results, your theme probably has a query_posts() call in the search results template. That throws Relevanssi off. For more information, see The most important Relevanssi debugging trick.