This is the code for the guide: http://extracatchy.net/wordpress-search-form-bootstrap

searchform.php

<form role="search" method="get" class="search-form" id="searchform" action="<?php echo home_url( '/' ); ?>">

    <div class="input-group">
    <input type="search" id="s" class="form-control search-field"
            placeholder="<?php echo esc_attr_x( 'Search…', 'placeholder' ) ?>"
            value="<?php echo get_search_query() ?>" name="s"
            title="<?php echo esc_attr_x( 'Search for:', 'label' ) ?>" />
    <div class="input-group-append">
    <span class="input-group-text"><button type="submit" class="search-button"><i class="fa fa-search"></i></button></span>
    </div>
    </div>

</form>