People in general are interested in people who are interested in them, and like to talk about that they are good at if they feel you are genuinely interested.
Make an effort to be genuinely interested in things that your ex is interested in. For example, if your ex is into gaming, see if you can genuinely be interested in gaming on your own – learn more about it, research, watch YouTube video/live streams etc., and when next time they mention ‘gaming’ in a conversation, tell them how you are learning about gaming, what you find interesting about it etc. and ask questions in a field where they think they’re an expert.
Just remember that your ex knows you, and they know when you are faking it – and will turn them off.