To teach the please concept to an intermediate signer when your baby makes a request (e.g. milk), acknowledge the request and ask them to say please (while doing the sign). "I'll get you milk. Can you say please?" Even if your baby does not understand at first, they will learn to copy. Then reward them by fulfilling their request. Please is great, even once children are older and are no longer actively signing. Use it as a secret code to gently remind your children to say please when your are with company, without causing them any embarrassment.
Conceptual baby signs like please are not well-suited for teaching using the flash cards. (We have made the flash cards for completeness, since lots of parents use the flash cards for other things like learning the signs themselves or to start teaching reading and writing.) Please is easier taught in real-life situations, having your baby practice the sign when they want something, before you give it to them.