[M] 20 first threesome

I am fucking with this girl(f21) for a week now, before that we chatted for a few weeks. We have a great chemistry and the sex is literally perfekt. She has a female best friend (f22) with which she had sex a few Times too. Now we had the idea to try a threesome with her, and she is up to it :D any advice for that? It is going to be my first one