There's no instruction on it!
Could someone write one? :)

Thanks in advance!

We here at ScummVM central do not write documentation, damn it :)

Seriously, if anyone wants to write some, feel free.

As for using Descumm(3/4/5/6):
DeScumm decodes the scripts themselves, so the FIRST thing to do it extract the script. You can do with several ways, such as playing the game with ScummVM and the DUMP_SCRIPTS define set in the Makefile/Project Settings.

Or you can export the file using ScummRev.

The resources that are considered valid scripts are things like:

OBCD
LSCR
SCRP