I put it in an envelop on the bed or hand it to her when I see her.
Some places leave an envelop for this purpose.
I usually leave it on the middle of the bed. They seem to get it.
You could also leave it on a table on top of a piece of paper and write on the paper "TU Propina, Gracias!" (Your Tip, Thank You!)
But it would be more gracious to write "Su propina, gracias señorita."
I always just leave it on the pillow, as I have done in many countries. Works fine.
Leave on pillow. They know it's not spare change.
I eiter leave it on the pillow or the bathroom counter. At the end of my stay I always like to leave a tube of Victoria's Secret Yummy hand lotion along with the tip. I learned the unexpected way many years ago not to leave spare change in ash trays.
how much is it custom to leave?
We prefer to tip daily and in USD for several reasons.
They too get days off or family members become ill, which prevent them from making it to work and that may coincide with your departure date. On occasion, one gets a senorita that is clever in making a variety of animals, flowers, and even hearts with towels (as clowns do with balloons) – that is always a bonus, which brings smiles upon our return after a long day away and lastly, we believe it keeps them honest (small price to pay for insurance). We have been fortunate in all our travels to not have lost anything from our room. However, we do take precautions.
Tipping in USD over pesos allows them the opportunity to hang on to them until the rate is most favorable.