Set the environment variable REMIX_TOKEN to your licence key from the Remix dashboard. If your site repo is not public, you could edit .npmrc and hard-code the value, but be careful to not commit it to a public repo.
To run the Remix dev server, run:
$ npm run dev
Open up http://localhost:3000 and you should be ready to go!
In the Netlify dashboard, set the environment variable REMIX_TOKEN to your licence key from the Remix dashboard. Then when you deploy to Netlify, everything should work.