f?p=app:page:session::YES/LEVELx:::
-- x is between 1 (least detailed) and 9 (most detailed)
f?p=100:1:&APP_SESSION.::YES
f?p=100:1:&APP_SESSION.::LEVEL5
-- OR
APEX_DEBUG.enable(p_level)
-- OR
APEX_SESSION.set_debug(p_session_id,p_level) -- p_level: NULL to disable