It's a bit counterintuitive, but you push! If you're on route /one/two/three and want to pop back to /one, you can just call either Routemaster.of(context).push('/one') or Routemaster.of(context).replace('/one'), depending if you want the user to be able to go back on the web. There's no popUntil method.

Call Routemaster.setPathUrlStrategy() on startup, before you call runApp():

void main() {
  Routemaster.setPathUrlStrategy();
  runApp(MyApp());
}