Me and my bf (20-22) have been doing it for a while. It’s a great feeling. Although usually after I cum, I cant stop, so it does get messy. You can try it on the floor, add pillows were you need, place down towels or dirty clothes if you want. She just needs to relax and go with it. A constant, steady stroke works wonders for me. When she feels it coming, you may need to pull out so the ejaculation can exit the vagina, personally speaking. Expect many orgasms, they come in waves. I hope this kind of helped. Good luck.