The trick is not to hurt her, at least not too much.
You want to excite her. The point is to make her see how much she wants you and how willing she is to behave in order to get you.
You should start by telling her, “I need to punish you.” This lets her know that you are in charge, you are not asking her, you are telling her what needs to happen next. What will happen next. A girl will respect you more for telling rather than asking — she always has the opportunity to say no and this way she doesn’t have to do all the work (she is already carrying her weight in the parts of your relationship you don’t notice).
Anyways, back to the punishment.
Most girls don’t really want to experience pain, so that should be avoided. A tug on her hair or even a hard slap on the ass doesn’t hurt, the body wants that, it translates as pleasure.
Let her know you are doing what you are doing for her pleasure too. You like being treated this way don’t you? It’s all the social conditioning women get to be people pleasers, it makes us feel happy when we make you happy, even when you’re being kind of a dick about it.
If possible you should bend her over something and not let her know whether you are going to spank her or fuck her. Suspense builds the best orgasms. A desk communicates power — that’s an aphrodisiac. A couch or table are more casual and plain, but they’ll do if they are the only option.
You should continually compliment her, or the role play might get to be too much. Remember, she is doing this for you. Tell her how beautiful you think she is, how hot she looks in whatever position she’s in, how turned on she makes you.
Be rough and honest about what you want her to do. Don’t make her do the work of being a mindreader. Don’t say things you think will sound cool that don’t come from your own mind. Going too far can cause her to think you’re insecure and overcompensating, do what comes naturally, don’t try to act like someone else.
Pay attention to how she responds to your hands. If her eyes are closed and she is no longer interacting with you, she’s not into it. She should be making eye contact as often as possible, looking at your face to try to read your expression and see if you are pleased with her.
When she begs for you to let her cum, hold out for only a very small amount of time before you let her. This is her reward for accepting what you give her.
Hold her afterwards. The whole point is that you have to trust each other for it to work. There’s not many places in life left where we demonstrate actual vulnerability and trust — where we place our physical and emotional safety in the hands of another. It’s very luxurious to be able to trust someone and to feel trusted. Savor that feeling.