Especially if you're getting more than one service, divvying up the tip can be a challenge, which is why many spa owners are happy to take over for you.
Just tell the owner what percentage you want to tip, and they'll add it on to your bill and distribute it to everyone for you. Your therapist will most likely be with a new client by the time you're ready to leave, so it wouldn't be possible to hand her a tip in person anyway.
The standard is usually 18%, 20% if you were particularly impressed with the service. It doesn't hurt to ask whether the gratuity is included, though. Some spas include it in the price of the treatment.