Mmmmmmmmmm - M is the mammalian letter, making your mouth mime the motion associated with mother's milk and infant comfort. Little wonder that m stands for mummy, maman, mama mia and childish names for mother all over the world.
This happy association makes m a relatively easy letter to teach, especially in the initial position. The main difficulties come in words such as jump and empty, where children who suffer from frequent upper respiratory tract infections often don't hear (or feel) the nasal m in a medial position. Plenty of practice with Humpty Dumpty (combining auditory, visual and articulatory awareness) can help.
Otherwise, the only things to remember about m are that it sometimes doubles (common, summer, mammal) to keep a preceding vowel short, and in one useful word from the Ancient Greek it is silent (mnemonic).
Mind you, it's a foolhardy claim that teaching any aspect of phonics and spelling is easy. M also appears in one of the consonant blends - sm, which seems straightforward until you see it from a five-year-old's point of view.
A teacher I met asked one of her Year 1 class to find three "sm pictures" in a set of cards (the targets were smoke, Smarties and a smiley face), and was surprised when he returned clutching pictures of a ladder, a car and a woman. However, as the little boy explained, these were indeed sm pictures:
"That's sm'ladder, that's sm'car and that's sm'mum." Mmmmmmmmmm.