Back in the day, there used to be envelopes in hotels that had somethng written on the front about being tips for the maids.
(I am 46, but I remember this from my teen days and staying in hotels at the Jersey shore).
I have not seen any envelopes in a long time, probably due to cost of supplying them.
We just leave a few bucks on a dresser or desk of our room. For me, usually, I leave our tip at the end of our stay because I will usually put a notice out that we do not need maid service. If I need new towels, I usually see the maids at some point and just ask for them.