To enable Memcache Storage support you have to add these lines to the end of your settings.php file:

Drupal 7:

$conf['cache_backends'][] = 'sites/all/modules/memcache_storage/memcache_storage.inc';
$conf['cache_default_class'] = 'MemcacheStorage';

Drupal 8:

$settings['cache']['default'] = 'cache.backend.memcache_storage';

The lines above are enough to start using memcached as a cachestorage. However, in a Drupal 7 world you might meet issues with "form" and "update" caches, because they are not types of cache which you want to store in memcached. So optimal basic config will be the following (Drupal 7 only):

Drupal 7:

$conf['cache_backends'][] = 'sites/all/modules/memcache_storage/memcache_storage.inc';
$conf['cache_default_class'] = 'MemcacheStorage';
$conf['cache_class_cache_form'] = 'DrupalDatabaseCache';
$conf['cache_class_cache_update'] = 'DrupalDatabaseCache';

That's all you have to do. Nevertheless, Memcache Storage has a lot of features which you might want to set up to get the best performance. Read other related topics to learn about them.