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 museum sign when you are preparing to visit an actual museum, whether for art, science, or other interesting themes. You can also teach this sign in the context of visiting virtual museums online.
This is a useful flash card taught best in conjunction with art, music, science, or geological flash cards. Of course, nothing beats having an actual museum experience, so try to arrange for a visit to a museum with your toddler as soon as they start showing interest in art work or artifacts from various disciplines.