How we difine user defined Exceptions in PL/SQL(Oracle)?

How we difine user defined Exceptions in PL/SQL(Oracle)?
User defined Exception
DECLARE
a NUMBER;
b VARCHAR2(10);
user_ex EXCEPTION;
BEGIN
if a > b THE
RAISE user_ex;
end if;
EXCEPTION
WHEN user_ex TEHN
dbms_output.put_line('User Defined Exception');
END;
/