The letter F is signed by holding up your dominant hand, palm facing out, with your index finger and thumb touching, while your 3 other fingers are held up and spread apart.
Make sure to form a circle with your thumb and index finger touching. Otherwise, if you flatten out your thumb and index a bit more, it will look like you are signing the number '9'.
Teach your toddler the letter F 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.