Hi Phil, this is how I would approach it:
admin/customers_csv.php (this is the main file which will display your data)
admin/includes/boxes/extra_boxes/customers_csv_tools_dhtml.php (the keyword 'tools' is important here, it instructs which menu the mod is shown under)
admin/includes/extra_datafiles/customers_csv.php (define your database tables used by the main file)
admin/includes/languages/english/customers_csv.php (define your language constants for the main file)
admin/includes/languages/english/extra_definitions/customers_csv.php (define mod's title/filename)
Look at my Zip Validator mod as it uses the same structure.
In your main file, you want to put these at the top and bottom:
PHP Code:
require('includes/application_top.php');
require(DIR_WS_INCLUDES . 'application_bottom.php');