We didn't do baby sign language, but I taught DD to gently pat my chest if she wanted a bf at around 6 mths. I got sick of not knowing what she wanted if she woke during the night. If she was whingeing for something I would pat my chest with her hand and ask her 'booby?' If she was eager, then I'd bf her. Didn't take her long to get the idea to just pat me herself whenever she wanted it. She learnt within a couple of weeks, from memory.

So I would suggest helping her make the sign for 'milk' right before bfing her. You probably know when she wants it anyway, given you've been bfing her for over 12 mths! It will just mean she associates the sign with a bf, not just hand actions that get Mummy excited.