If you've got a bug to report or a feature to request, just create an issue.
Pick an open issue you want to fix -- create one first if there isn't one.
Fork the repository and write your code.
Launch with npm start.
To get changes more easily, run npm run watch in one console and npm run exec in the other.
Refresh the window to get updated code.
Make sure your code passes the tests (npm test).
Also, make sure it actually works.
Submit a pull request.