The sign for pee (as in needing to urinate), is made by tapping the ASL sign for the letter 'P' on your nose, as if you are discreetly asking your baby if they need to go to the potty.
Sign the letter 'P' by making your dominant hand into a fist, with you pointer finger up and your middle finger extended at a 90-degree angle. Then tap your middle finger on your nose.
Starting to communicate with your baby about when they are about to pee and poop is an important first step to potty training. Begin by making the sign every time they pee, and gradually they will start making the sign every time they are starting to feel the need to pee.
Pee is best initially introduced in the real-life situation, but adding the flash card will help your baby generalize - and can help overcome the problem of your baby only learning to make the sign after the event!