How can you make an open-relationship work?

You have to make sure that both people definitely want an open relationship. It's not something that you can push the other person into, because they'll be miserable and, if you're at all a decent person, that will make you miserable as well. Other than that: communication, respecting boundaries, communication, honesty with all your partners, communication. If you're not fond of talking about your feelings, I can't recommend an open relationship, because you'll end up doing it a hundred times more often.