Windows:

Copy a .vst3 file to
C:\Program Files\Common Files\VST3

Copy a .dll file to your VST2 location. Common locations are:
C:\Program Files\VSTPlugins
C:\Program Files\Steinberg\VSTPlugins
C:\Program Files\Common Files\VST2

Or use a custom directories configured in your DAW’s preferences.
Restart your DAW and rescan plugins if needed.



macOS:

Copy the files to the correspondent locations:
/Library/Audio/Plug-Ins/VST3
/Library/Audio/Plug-Ins/Components

Open Terminal and run the commands:
xattr -cr /Library/Audio/Plug-Ins/VST3/Gneiss.vst3
xattr -cr /Library/Audio/Plug-Ins/Components/Gneiss.component

Restart your DAW.

If the plugin still doesn’t load, check your DAW’s plugin blacklist and re-scan.
Note that some DAWs (like Logic Pro) may refuse to load unsigned plugins regardless of these steps. Others (like REAPER) are more permissive and will work fine.
Please note that you have to re-run these commands after you download a product update.