Elevated Site Search replaces the results of the default WordPress search automatically: activate the plugin and every existing search form on your site starts returning typo-tolerant, relevance-ranked results from the search index. There is no setting to switch on. This page shows how to verify it and explains exactly what changes.
Install and activate Elevated Site Search (see Installing & Activating). Activation schedules the first index build automatically; on most sites it finishes within a minute or two.
You can watch the build under Elevated › Site Search › Settings › Advanced Options › Indexer.
Use any search form your site already has, or open yoursite.com/?s=denim directly, and misspell the word on purpose: search for denmi instead of denim.
The results page returns the right content anyway. Results come from the search index, ranked by relevance instead of publish date.

When a visitor submits a search, Elevated Site Search intercepts the main WordPress search query and answers it from its own index: the same index, scope, and fuzzy matching the instant search box uses. Your theme keeps rendering the results page; only the list of results changes.
Searches that return nothing produce a normal “no results” page from your theme.
Search results look unchanged. The index may still be building. Check Elevated › Site Search › Settings › Advanced Options › Indexer, and rebuild it from there if it is empty.
A post never shows up in results. Its post type is not in the search scope. Add the post type under Settings › Advanced Options › Content to Index; the index rebuilds automatically when the scope changes.
The results page title shows an empty search term. This is a known issue in the current release: the page works, but templates that echo the searched phrase render it empty. It is tracked and will be fixed in an update.