Most phonics systems begin with three letter words like “dad” and “pat.” You could just as easily begin with two letter words like “at” and “ab.” Don’t worry if the word isn’t real. A four- or five-year-old child won’t recognize that some words are not real. If the child seems perplexed, explain that “ak” is not a real word. But if the child doesn’t question a word, as long as he pronounces it correctly, skip the explanations.
I have found that skipping the two-letter short-vowel words is a mistake. If a child becomes used to always seeing a consonant at the beginning of a word, he might become confused if a word starts with a vowel. If a child knows his sounds and letter correspondence, then there is no reason why two-letter words should confuse him. To avoid this problem, I would not wait to teach two-letter words.
When the child has successfully combined most of the first handful of letters into words, add more consonants but keep the same vowel. “H,” “j” and “l” are good second choices. You might think that “h” has different sounds when combined with “c” and “s” (“ch” and “sh”). True. But as a first letter, “h” always sounds like an “h.” The sound of “h” has a one-to-one correspondence to the letter “h” at the beginnings of words; it is distinct. The child won’t be confused because no words end in only “h.”
How long does this take? Some kids pick up the “code” of reading almost intuitively but for others it’s a long struggle to learn. Don’t pressure a child to move on if the child isn’t ready.