WordPress.org

1. Upload this plugin to your WordPress plugin directory( generally wp-content/plugins/ ).
2. Activate it in admin area.
3. Your theme should have a hardcoded search form or you should add a "Search" widget to sidebar to allow visitors searching.
4. Check whether or not there is a "searchform.php" file in your current theme directory, if not, you don't need do anything; If yes, edit this "searchform.php" following the below instructions:

Your theme's "searchform.php" file may looks like below, with some other codes:

....
<form method="get" id="searchform" action="<?php bloginfo('url'); ?>">
    <input type="text" id="s" name="s" value="<?php the_search_query(); ?>"  />
    <input type="submit" id="searchsubmit" value="Search" />
</form>
...

add the <?php if(function_exists('esf_add_form_field')) esf_add_form_field(); ?> code to proper place inside <form> element, in most cases you can put it just after <input type="submit" value="Search" /> button and before </form> close tag.

That's all. You may want to add css styles to make search form nicer.