FUNCTION MODULE

I WOULD LIKE TO CREATE A USER DEFINED FUNCTION MODULE .

MAY I KNOW THE PROCESS TO CREATE OUR OWN THE FUNCTION MODULE??

AND WHAT FIELDS ARE MANDATORY WHILE CREATING THE FUNCTION MODULE LIKE EXPORTING ETC...

IS THERE ANY T-CODE TO CREATE OUR OWN FUNCTION MODULE ??

ELSE SHOULD I USE CALL FUNCTION 'FUNCTION_MOD_NAME' ??