It depends.
This might be a LONG answer, bear with me.

You need to start with what exactly turns them on. If you and your gf are not very communicative about it, there is still go the trial and error way. Speaking of examples, here are three I can give.

1. A - She was a vanilla lover. To her, dirty talk consisted a lot of poetic messages.

   • Saying her name with a sense of helpless urgency
     
   • Telling her how badly I need to undress her and make love while fooling around
     
   • Showering her with compliments while fondling her or leading her to bed
     
   • Telling her out loud "oh that's so sexy", whenever she does something you find sexy
     
   • Describing how hard she makes me, how hard I am. Touching her and describing to her how wet I can feel her to be and how excited that makes me.
   • From simple “Oh you like that?” to “Oh yeah, take me in” etc.
     
   • Whispering the climactic things (I'm gonna cum/I want to see you cum) etc.
     
   • A lot of "i love you"s along with "i need you"s and "you are so hot/you drive me crazy"

2. B - She was a Milk-Chocolate lover. That's a little darker, with things promising to go wild on occasion.

   • Describing in detail how I’d go about undressing her. Describing which of her lingerie you’d tear off how etc.
     
   • Telling her how I enjoy making her nipples erect, her clit sensitive and her pussy moist.
     
   • On occasion calling her dirty names (Bitch/slut) etc. making sure she was comfortable about it(this needs communication earlier)
     *Phone sex comes in play! Naughty texts etc. Telling her about I wish I could just walk into her place of work, take her to some store room, push her against the wall and take advantage of her while she suppresses her moans.
     
   • Conversely, taking her out in the nature and how I’d make her scream out loud that no one can hear and she can let herself go etc.
     
   • Discussing fantasies and talking about how to go on about those. A bit of roleplay, master-slave etc. comes along with their own terms and rules.
     
   • Communicating about where I am going to cum on her. Or how I’m going to kiss her/grab her/cling to her/spank her as she cums.
   • Sometimes, just telling her what to wear, expressing your hunger for her flesh while out in public where you know nothing physical can happen would be enough.
     

3. C – She was a DARK chocolate lover. Submissive, eager to please and most responsive of them all.

   • The dirty talk gets FILTHY at this point. Bitch/Slut/Whore, conversely, Master, Daddy etc. nothing is to be held back.
     
   • Spanks, clamps come into play and are used while the words get humiliatingly dark. Randomly telling her to get on her knees and “Suck my dick like an obedient little whore” to “Shut up and open your legs, I am hungry” and then proceeding to eat her, sometimes when she least expects it are all part of the magic.
     
   • The fantasies are endless and merely telling her how to sit, lie down or how to undress herself gets her wet. Making her sit close by and commanding her, telling her how to touch herself gets your dirty talk covered. Look at her cues and control her orgasm. Deny her pleasure/grant her pleasure, all part of the package, with just words involved.
     
   • Sometimes just call her up in the middle of the day, and describe the fantasy where she gets home, she is to undress naked right at the door and crawl on her knees to find me and stroke me while telling me about her day. Casual talk while the the actions are sensual.
     
   • Make more n more of such dirty plans and keep TALKING to her about it. And when she gets all worked up, have her act on it. Making her thank me for all the excitement I give her etc.
     

Finally, remember, after all the dirty talks are finished and orgasms are done, the intimacy is NOT finished. The key part comes right AFTER the rush of the orgasms and filthy talk subsides. THAT is when you connect. Read your gf’s body language to see if she wishes to discuss what just happened. Take mental notes on what happened, what she liked/responded the most to. And if she is shy and won’t discuss outright, just remark “you really liked when I <name of sexy move>, huh?” and if she blushes, she loved it.

Again, sorry about the long message. Let me know if you have any questions!