The letter W is signed by holding up your dominant hand in a fist, palm facing out, with your index, middle, and ring fingers sticking out. This looks just like the letter 'W'.
Be careful not to confuse this with the number '3' in ASL, where your thumb sticks out instead of your ring finger.
Teach your toddler the letter W sign when it is necessary to finger-spell some simple words - or the initials of objects, places, animals, or people's names. That way, your toddler slowly begins to associate the sign with the sound of the letter in a word or name.
This is a useful flash card taught best in conjunction with the other letters of the alphabet.