Do what you normally do when you're building any kind of relationship, work out what you want, work out what they want, give them what they want so that you get what you want.
If you're doing it for purely selfish reasons, and sex is your only motive, you shouldn't have a problem, make sure she likes you enough, get her drunk, whatever you feel you need to do.
Why do people have a such a problem with casual relationships here?