Custom Report Template
Not sure which one you used - but this is still one of the best guides out there: 

https://alastaira.wordpress.com/2011/03/17/civicrm-creating-a-custom-report/

Note: the .php goes into your custom_php and your .tpl goes into your custom_templates (and you must set these paths in the Administer -> System Settings -> Directories: Cutom PHP Path Directory and Custom Templates).

After adding these paths and putting the .tpl and .php into place:

Clear caches: Adminster -> System Settings -> Cleanup Caches and Updates Paths - and hit the Cleanup Caches button.