WordPress.org

1. Extract all files from the ZIP file, and then upload the plugin's folder to /wp-content/plugins/.
2. If your blog is in English, skip to the next step. If your blog is in other language, rename the file stopwords in the plugin directory as something else or remove it. If there is stopwords.yourlanguage, rename it to stopwords.
3. Activate the plugin through the 'Plugins' menu in WordPress.
4. Go to the plugin settings and build the index following the instructions there.

To update your installation, simply overwrite the old files with the new, activate the new version and if the new version has changes in the indexing, rebuild the index.

If you want to use the custom search results, make sure your search results template uses the_excerpt() to display the entries, because the plugin creates the custom snippet by replacing the post excerpt.

If you're using a plugin that affects excerpts (like Advanced Excerpt), you may run into some problems. For those cases, I've included the function relevanssi_the_excerpt(), which you can use instead of the_excerpt(). It prints out the excerpt, but doesn't apply wp_trim_excerpt() filters (it does apply the_content(), the_excerpt(), and get_the_excerpt() filters).

To avoid trouble, use the function like this:

<?php if (function_exists('relevanssi_the_excerpt')) { relevanssi_the_excerpt(); }; ?>

To uninstall the plugin, first click the "Remove plugin data" button on the plugin settins page to remove options and database tables, then remove the plugin using the normal WordPress plugin management tools.