Want to have sex with a girl but nothing more

I don't know how you'd go about approaching it. It arises organically when two pragmatic, mutually-attracted people want sex without a relationship. You don't really approach it with someone. You feel your way around it, experience chemistry and just go with it.

Best to find a person who you want it with and who wants it with you first. If you're abstractly wanting it with anyone, you're probably going to end up treating them less like a person and more like a sex portal.