You don't. Not if you wanna get the tang. Actually, even if its just a sex relationship, its still a relationship. So you don't really have to lie to her.
Give her your number. Let her call you, set up a time to hang out with her. Bang her. Always be respectful of her, and bang her some more. Don't call her, unless it is to "hang out" which leads to banging. Also, talk about her feelings when she wants to. Then bang her.
When she wants to talk about your feelings, or where you feel the relationship is going... Tell her "I think things are going good." A great way to dodge the bullet if she directly asks you if you two are exclusive is to accuse her of sleeping around on you, but with some humor. If she keeps pressing the buttons, tell her you aren't with anybody else right now (Which isn't a lie. At that instant, you would be with her.)
After that... Bang her.
Pretty soon she will develop an understanding that when she sees you, it means sex.