Study the existing reporting/repXXX.php files and the variable input form parameters file reporting/reports_main.php.

If you wish to have your own class of reports (Reports and Analysis Menu), then append it to the first stanza in reporting/includes/reports_classes.inc:

// Standard report classess
define('RC_CUSTOMER', 0);
define('RC_SUPPLIER', 1);
define('RC_INVENTORY', 2);
define('RC_MANUFACTURE', 3);
define('RC_DIMENSIONS', 4);
define('RC_BANKING', 5);
define('RC_GL', 6);

define('RC_PAYROLL',7); // <------ Added now