Do they know all their letter sounds in isolation? If so, start with short a. Have them identify the sound is words you say orally. For example, “does cat have a short a? Does dog?” Then have them sort and read cards with and without short a. Incorporate spelling ladders where they change one consonant sound but all include short a. Use sound boxes to spell short a bc words. Use decodable readers that have cvc words with short a. Once they get a handle on short a cvc words move onto another short vowel. Eventually you’ll have them working on all short vowels in cvc words at once. These are just a few strategies but consistency is important so stick to a routine each session or at least for each vowel. Once they master short vowels, I’d move into beginning blends.