How do I define a template file for a specific node ID? I'm trying to define a custom template file for a specific node, using the node ID.
I read a lot of topics saying to create a node--X.tpl.php file in the templates folder.

That's what I did; unfortunately, even after flushing the theme cache many times, nothing changes.

Is there anything more I have to do?

I am using Drupal 7.
In Drupal 7, node--[node-id].tpl.php is used as template file for nodes, as well as node--[node-type].tpl.php. Nothing else needs to be done, except, as Clive wrote in his comment, you also need to have node.tpl.php in the directory containing the template files used by your theme.

Notice there is a difference between node--[node-id].tpl.php, and page--node--[node-id].tpl: The latter is also used for pages at node/[node-id]/edit, or node/[node-nid]/delete. See the comment at the beginning of theme_get_suggestions().


  Build a list of suggested theme hooks or body classes in order of specificity. One suggestion is made for every element of the current path, though numeric elements are not carried to subsequent suggestions. For example, for $base = 'page', http://www.example.com/node/1/edit would result in the following suggestions and body classes:


 page__node              page-node
 page__node__%           page-node-%
 page__node__1           page-node-1
 page__node__edit        page-node-edit