Mage tipping etiquette?

I'm getting a mage for the first time and I've read that it's customary to leave a 15-20% tip. This is a relaxation/deep tissue mage, not one with a happy ending lol. My question is where do I leave the tip? Should I hand it to her or do I leave it somewhere? Should I give it up front when I pay? Anyone who's done this before please help me out. Thanks!