Find someone with the Dom, Sub, or Switch inclination. You can do this by using the Findr app, cheating to look at their inclinations, or going on dates/hangouts with them to learn more about them. You can also tell another way: Approach them to talk through the People tab. Choose the flirt option. If your Assertiveness/Dominance or Compliance/Submission is high enough (around 2 or 3) you should have the option to flirt dominantly with a submissive person, or flirt submissively with a dominant person.
Date the person a lot and act the role you want. For example if you want a Dom let them pay for dinner and movie dates, let them pick your food during dinner, let them touch you or speak up for you in the movies, let them lead during sex, send them pics when they ask, etc. If you want a sub you should be the one paying, speaking up for them, fondling them, moving them and being rough during sex, or sending pics.
Go on enough dates and act the role long enough and the person should eventually get a black bar under their Romance bar that is either Submission or Dominance. Keep dating and get that bar super high. Eventually you can approach them in the People tab and choose to change your relationship status with the person, and it should be an option to make them your sub or dom (although if they like you enough they may approach you first!)
Note that this process can take a while, it usually takes me a month or more of ingame time. Also note that a Dom/Sub relationship is considered a ROMANTIC relationship, so if you're in an exclusive romantic relationship with someone else (boyfriend/girlfriend or open partner) there won't be an option to have a Dom/Sub. You can have a Dom/Sub and any amount of poly partners, though.