How to make WP Query Pagination work

try this:

$the_query = new WP_Query( array('posts_per_page'=>12,
                                 'post_type'=>'customers_gallery',
                                 'paged' => get_query_var('paged') ? get_query_var('paged') : 1) 
                            ); 
                            ?>
<?php while ($the_query -> have_posts()) : $the_query -> the_post(); ?>

  //YOUR CONTENT YOU WANT TO SHOW

<?php
endwhile;

$big = 999999999; // need an unlikely integer
 echo paginate_links( array(
    'base' => str_replace( $big, '%#%', get_pagenum_link( $big ) ),
    'format' => '?paged=%#%',
    'current' => max( 1, get_query_var('paged') ),
    'total' => $the_query->max_num_pages
) );

wp_reset_postdata();