and how do I become someone's dom/sub?
Find someone with the submissive or dominant or switch inclination. Go on lots of dates with them and build up their dominance/submissiveness meter (Let your dom order food for you, fondle you, speak up for you, etc. Or be the one to speak up for/fondle your sub) After lots of dates and if your own stat is at least 3 (3 submissiveness/compliance if you want a Dom, 3 dominance/assertiveness if you want a Sub) approach them to talk in the evening. You can propose a relationship or they might propose one first. It can take a long, long time tho. Like a month or more of in game time.
Yes, but you won't know right off the bat what a stranger's sexual inclinations are (unless you have cheats turned on), you have to get to know them. If you're looking specifically for a dominant or submissive person and don't wanna cheat or date around a bunch you can use the Findr app on your phone to find people with those inclinations right away.
Try being more dominant on dates (speak up/fondle them in the movie theater, pay for dates, be the one moving them around and controlling the action during sex, find them in the evening and flirt dominantly if your skill is high enough, etc) and eventually a submission meter will appear under their romance stat.
It definitely could take a while! It took me a couple months ingame time to get a Master, for example. I think it goes faster if they have a naturally submissive personality (shy/insecure/passive people are more likely to be your sub then forward/aggressive/confident people) and of course they need the Submissive or Switch inclination.
A Dom/Sub relationship IS a romantic relationship, just with added mechanics. So if you're in a monogamous relationship (with a boyfriend/girlfriend or open partner) you can't also have a sub. But if you have the polyamorous inclination you can have as many poly partners and doms/subs as you want. You get the polyamorous inclination by starting with it, completing the Best Friend questline and choosing to be poly with them, or by cheating and giving yourself the inclination.