The thing to remember is this kind of thing relies *heavily* on communication between the two or more individuals involved in the scene. I would strongly recommend asking the other person what it is exactly they're wanting. You know they're wanting a spanking from you. Do you know if they want to be over your knee, laid out, bent over, etc? These are the kinds of questions that would be really good to ask.

As far as broaching the topic, the way I personally would do it would be to let them know you want to talk about the scene they're wanting to do, and that you have a few questions.