The letter M is signed by holding up your dominant hand, palm facing out, with all your fingers curled into your palm. Then tuck your thumb between your ring and pinkie fingers. This sign looks like the small letter 'm', with the 3 fingers' bumps reminding us of the letter written in cursive.
Teach your toddler the letter M 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.