Hi, and welcome. It sounds like you have everything in place already! You're friends. You're comfortable together. You both want the same things in a spanking scene. You both have a degree of experience. Your lines of communication are open.
You might want to discuss limits, and safe words, if you haven't already. If you're into scenarios/roleplays, you might want to bounce some back and forth and see if any appeal. Ask her to share her fantasy scene. Otherwise... I would say just relax, get together and have fun! If you both like videos, choose one to watch, if that helps you get into the mood. Enjoy each other, laugh, relish the experience.
My first time was nearly 12 years ago, and I think the only thing I would have changed was asked for more holding/hugging/caressing afterward. But the spanking itself was amazing, his demeanor was perfect and he asked all the right questions, and I wouldn't have changed any of that.
Hope you'll tell us how it went!
-- Erica