https://www.nexusmods.com/subnautica/mods/201/
https://www.nexusmods.com/subnauticabelowzero/mods/1/
For instructions on how to create a mod, see the wiki.
The log file for QModManager can be found in the base game folder with the name qmodmanager_log-Subnautica.txt or qmodmanager_log-SubnauticaZero.txt, depending on the game.