Change slug with custom field

The slug is saved on the wp_posts while custom fields are on the wp_posts_meta. If you want to make it like that you can use an action hook on save_post that will get the value of the custom fields and saves it as the post slug.

Here is the code

add_action('save_post', 'set_slug');

function set_slug($post_id){
    $new_slug = get_post_meta($post_id,'custom-slug', true);    
    $post_args = array(
        'ID' => $post_id,
        'post_name' => $new_slug,
    );

    wp_update_post($post_args);
}