To sign letters, form the ASL L sign with your dominant hand. Then form a fist with your non-dominant hand while only your index finger sticks out. Touch your non-dominant index tip to the tip of your dominant thumb a couple of times. It is as if your pointer finger is drawing your attention to the Letter L -- and therefore any other letter you teach your baby.
Teach your baby the letters sign when you start teaching them the alphabet. Eventually, you will be able to sign the entire "ABC" rhyme while signing with and for your baby. One day, they may just surprise you by singing the entire alphabet in one go as well!
This is a useful flash card taught best in conjunction with all the alphabet flash cards, when your baby is ready.