For a newbie, check her website and follow her instructions. If she says "Plain, unmarked, unsealed envelope" go with that. Some say that a birthday card (don't write anything, just use the card and unsealed envelope as a money holder) is OK.
Does her website specifically say to leave it in the bathroom? (You go to wash your hands, leave it on the sink or toilet. You exit. She goes in to wash her hands and checks that all is in order.) Then leave it in the bathroom.
Does she host from a hotel? There are usually some easy, obvious places on the hotel furniture. As you take off your jacket, remove the envelope, walk over to the TV and slide it under the corner of the TV, just enough so that it doesn't fall and remains visible. Then, back to the place where you were hanging up your stuff.
The nightstand often has good spots for a safe landing: lean it up against the lamp base; slide it under the clock/radio just enough to keep it from falling but not enough to obscure it. If there's a small desk, I look for something on the desk to serve as a paperweight to keep it in place without hiding it.
If you are seeing her at a private place, you can probably find similar things but even more options. Many studio apartments have a kitchen counter of some sort that is part of the main room.
If she is neat and orderly and has removed her shoes and placed them by the door (instead of flinging one up to the chandelier and kicking the other shoe under the bed), you can tuck the envelope into her shoe. Well, I did, and there was no problem.
You can be obvious without being TOO obvious and you don't need to say anything or at least not anything specific. "I'm just going to leave this here." while tucking the envelope under the TV is usually OK. But you don't have to say anything at all.
Good luck