Clone this repository into a folder of your choice. In this example I cloned it into the ~/ folder.
Change dir to ~/.node-red.
cd ~/.node-red
Install this package with the command
npm install ~/node-red-contrib-discord-many-nodes
This will create a symbolic link to the node module project directory in ~/.node-red. Node-RED will discover the node when it starts.
Now Restart Node-RED:
node-red-restart
If you are using a service for Node-RED, then restart your service.