I'd recommend installing/creating a logging framework that allows you to set the logging level (debug/info/warning/error/...), then stores all higher priority to a database table.

For example, logger:

https://github.com/OraOpenSource/Logger

Then call this in your trigger body, recording all the old/new values of interest. e.g.:

begin
  logger.log ('Old values ' || :old.c1 || ', ' || :old.c2 ... );
  logger.log ('New values ' || :new.c1 || ', ' || :new.c2 ... );
  
  ... trigger processing ...
end;
/