If you want to install for all users, simply copy the file into the folder:
If you want to install just for your user, simply copy the file into the folder:
You may need to restart the QuickLook manager. To do so, you'll need to open a Terminal and execute:
If you installed it before updating to Catalina (OS X 10.15), you will need to install it again after updating to Catalina, due to new security features present on Catalina. Download the latest release and install as usual.
Here you have it QuickLookJSON.qlgenerator