As sur(name removed by moderator)ure notes, it matters what you mean by liberal, as that determines your approach. Personally, I think it comes down to keeping yourself as well-informed as possible and then being able to talk in a way that’s relevant to the person you talk with. For instance, the other day I got into conversation with an LDS (mormon) missionary. We avoided politics (I’m politically to the left, though moderately so, and it’s a safe bet most LDS people are politically right), but instead just talked about our respective faiths. I asked him specific questions about his, and encouraged him to ask me about mine. I was able to explain things like purgatory, and the centrality and meaning of the sacaraments to him. I think we both learned a lot.
On the other hand, living in Manhattan, I interact with a lot of people who are not only politically liberal, but often outright hostile to religion as well. With them I prefer to talk politics, such as health care and peace, topics which we share in common, and then bring in the fact that it’s because of my faith, not in spite of it, that I hold these positions (other issues such as abortion are trickier - a lot of emotion involved, and it’s difficult to find a common language, but I’m working on it).
Of course it helps if they’re actively seeking dialog. People who already know all the answers are, by and large, not interested in what you have to say.