You have to clone the source code to your local machine
Place it in the CustomWidgets Folder in a mendix project
Also go through the SCRIPTS section of package.json file. From what i see you need to run
npm run dev to have the scripts keep your files in watch mode while you integrate the widget into the mendix project and finish testing.
Once you are happy you can use npm run release to generate the mpk of the widget which you can distribute to anyone directly or publish to Marketplace.
Did you already try the above steps? if not, please give it a go and let me know if you are stuck anywhere. I can help out.