Easy. Get their attention by whatever means necessary.
Talk about things that are interesting to you. Ask them about things that they are interested in.
Actually listen and show genuine interest in the things that they like by asking questions and following up on things that they tell you.
If they do that for you too, great. Continue to build rapport, go on dates, fall in love, get married, build a life together, have kids, become happy.
If they don’t, don’t take it personally and just move on. If they’re not interested in you, then you should not be interested in them. Figuring out this part is hard, but once you do, you will save so much time and mental.