Teach vowel sounds in isolation first. Practice sounds as you introduce the letter name. Children have to understand that each letter has a corresponding sound. Review vowel sounds every day. Even second graders will benefit from a short daily drill of these sounds.
Provide opportunities for students to listen to sounds. They will benefit from hearing fluent examples. Besides hearing you pronounce vowel sounds, they can listen to sounds at A to Z Phonics, an educational site. Children click on videos to hear short or long vowels.
Reinforce vowel sounds when teaching writing skills. If you teach kindergarten, you write sentences on chart paper every day. Call on volunteers to circle vowels in the sentences then say the sound for each one. In first and second grades, dictate short sentences and have students circle all the vowels.
Decode, or sound out, words with vowel sounds. Children do this as they read, but they need a lot of practice to achieve fluency. Use a consonant-vowel-consonant word list. Write them on index cards and make several sets for classroom use. Students can work in pairs calling out words to each other and having their partner say each sound they hear in the word. This segmentation activity helps students hear each sound separately.