The middle row demonstrates 3 bad pairs of letters to use because each is too similar to the one next to it. It wouldn't be easy to tell them apart.
The bottom row introduces first lamda standing on two legs with a neck like a giraffe's
easily distinguished from theta, an egg split into two halves. Once a child could pick up the one I named, I'd introduce a third letter, phi, very unlike the first two, an olive on a stick. Once that was mastered, mu would make an appearance, as different from lamda as chalk is to cheese.
But of course, we're not teaching Greek so which English letters might you choose? For one of my children at 12 to 15 months we started with the wriggly s by drawing it in the air and hissing like a steam train, easily distinguished from the letter i which we drew in the air and clicked the tongue as we put the dot over the top and called it eye-dot. Soon after, the t for which we used a sibling's name, Tom. Once mastered, m for Mum made an appearance. And so on ...