They will probably phone first (about 9:30 or so at night) and ask if you want a massage. Say yes and someone will come up, maybe not alone. Sometimes the girl brings a "business manager" type person (aka mama-san or pimp.) Sometimes with a second girl to see if she can up-sell you into taking on a pair.
You will have to invite her in and explain what you want. Let her see your girlfriend. Be up front about it. This is not the time to be shy. If she says no, then it's no. Nothing lost. You can ask if she has a friend who might be OK with it.
Will definitely cost you more than a straight quickie romp because you are ordering an "off the menu" service instead of just buying the "blue plate special."
It is *not* a slam dunk. This is not the land of "golden unicorns." Also, if you can't speak Chinese, she will probably just be confused.