WP_Query_Multisite is a subclass of WP_Query, WordPress' post querying class. The class does everything behind the scenes, so the only change you make to query multisite is the in the class declaration expression.
Example usage:
$query = new WP_Query_Multisite( array( 'post_type' => 'post' ) );
while( $query->have_posts() ) : $query->the_post();
echo $blog_id . get_the_title() . "<BR>";
endwhile;
wp_reset_postdata();
To modify what sites are queried, create a sites key in the $args in the constructor parameter, with a sub-element of either sites__in or sites__not_in, which will be an array similar to posts__in in the WP_Query object.
Example usage:
$args = array(
'post_type' => 'post',
'sites' => array(
'sites__in' => array( 1, 2, 3, 5 )
)
);
$query = new WP_Query_Multisite( $args );
while( $query->have_posts() ) : $query->the_post();
echo $blog_id . get_the_title() . "<BR>";
endwhile;
wp_reset_postdata();