Drop this code to header.php (for example if you want show it on header), put your gallery ID in [slideshow=1] (when your gallery ID is 1)
<?php
$showgallery = '[slideshow=1]';
$showgallery = apply_filters('the_content', $showgallery );
echo $showgallery;
?>
Then, adjust your slideshow width and height on Nextgen Gallery’s slideshow option, and your layout css off course.
Good luck :’)

