You know how to smack lightly, don't you? Go on and slap your own (naked) bottom until it stings just a bit, and in a
good way

Then you'll know how much force to use. And, like Bigballs has suggested, start out light and gradually use more force, paying very close attention to her body language. If you aren't sure whether or not she enjoyed a spank, ask her.
Make it an erotic experience, though, don't just go at it *slap, slap, slap*. Grab her and tell her, in a seductive voice, that she's been a very bad girl and it's time for her punishment. Then forcefully put her over your knee and toy with her a bit before sliding her panties down. Pause between slaps and run your hand over her ass, her thighs, slip your hand between her thighs. Tease her lips with your finger, "examine" her pussy, bend over and kiss her ass, tongue her crack...all depending on how comfy she is with anal stimulation, you could segue into rimming and/or oral. (But don't go from the ass to the pussy--that's asking for an infection.)
Have fun!