I just struggled through this myself earlier today. I used the guide here: http://www.steves-internet-guide.com/securing-node-red-ssl/
I created the certs in the same directory as the settings.js file and used the paths listed below in the config.
key: fs.readFileSync('/data/node-key.pem'),
cert: fs.readFileSync('/data/node-cert.pem')
I would recommend un-commenting the "requireHttps: true" line in the settings file and make sure you add a comma after it.
Also don't forget to un-comment the var fs = require("fs"); line near the top of the config.
Best of luck!
HC