How to manually install mods for Subnautica and the below-zero stable branch: 
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Install Tobey’s Bepinex Pack.
Subnautica: [Subnautica Bepinex] (https://www.nexusmods.com/subnautica/mods/1108)
Below zero: [Below Zero Bepinex] (https://www.nexusmods.com/subnauticabelowzero/mods/344) -
Download Nautilus, the main API for Subnautica mods, and follow the installation instructions in its description.
Subnautica: Nautilus
Below Zero: [Nautilus Bz] (https://www.nexusmods.com/subnauticabelowzero/mods/373) -
For any mods you install after, they would have to be unzipped, then put into
Bepinex>Plugins