Hi
I got some of my overrides working and got the Idea I started to understand.
No I trie to override the jscript_form_check.php
in
/webshop/includes/modules/pages/create_account/
so I created
/webshop/includes/modules/pages/create_account/CUSTOM/
and copied the jscript_form_check.php from the original folder and editted that file
and also:

/webshop/includes/modules/pages/login/
so I created
/webshop/includes/modules/pages/login/CUSTOM/
and copied the jscript_form_check.php from the original folder and editted that file

When I replace the file in the original folder I can see my changes work.
But when I try them in the custom folder nothing happens:S

Where did I go wrong??