You will want to have a conversation ahead of time. Hash out what she wants to have done, what her limits are, etc. If she wants you to surprise her, proceed VERY cautiously. It's easy to cross a line.
Establish a "safe word". Regardless of what you're doing, if it's rough, you need to have a safe word.
If(hopefully) you guys discuss it, don't be afraid to throw out your own ideas. I've been with a handful of girls that preferred rough treatment and I quickly stopped being shocked at what they would agree to. Nothing absolutely insane. Just submission to a sometimes extreme level.
If I had to suggest a starter pack type thing for you, I'd get her consent to try.....
Calling her derogatory names.
During oral push her back and slap her lightly to begin with. If it's not hard enough, she can say so, and then you can go harder.
Same with sex. Slap/smack her on the face, the chest, the ass. If that's not enough, push it further for her.
Be rough and controlling with blowjobs. Force her head, gag her if she likes that, pull her hair, slap her again.
Put your hands around her neck. Careful. I'm not gonna suggest choking because "breath play" is too dicey/dangerous. At least to begin with.
Tie her up. Girls that like rough sex typically like to feel helpless. Bondage, even just hands behind the back, can really, really do the trick.
Order her around. Don't ask. Tell.
Throw her around some. Nothing crazy. Just forcefully place her where you want her to be.
There's a endless list of shit you can do. It all depends on what she's comfortable with or how far she's willing to push herself.
It won't feel like it at times(it can start to feel almost too empowering) but it's just, if not more, about her pleasure than it is yours.
Again, the key, THE KEY, to having a mutually good time with rough sex is to know where her limits are and to never get carried away and barrel past those limits.
It may feel kinda weird, intimidating at first to do that stuff to her. I felt that way the first time I got this kind of request.
Once I started working towards what she and I wanted, the better and better it got.