Variant vowels are vowel sounds that are controlled by the letters surrounding them. These vowel sounds may be confusing to some children, as they're typically spelled differently but make up the same sound. Children will need to master these to become proficient in spelling and reading, so it's necessary to help them understand with practice.
Songs can be a helpful way for some children to memorize different rules within the English language. If you can play a guitar, try learning a few songs about variant vowel sounds and teaching them to the children. If you can't play music, you'll still be able to teach songs by purchasing CDs that feature songs about variant vowels. Songs for Teaching and Reading Manipulatives both sell songs teaching about vowel sounds.
Draw pictures which represent the different vowels you're discussing to help children visualize what you're talking about. Show these pictures to the children as you're discussing them. For example, when discussing the "oo" sound, draw a picture of a moon, a tool and a tune (or a note to represent this). Go over these several times and then erase the pictures, asking the children to simply read the words.
You can make or purchase flash cards on variant vowels for your students. Or, you can ask your students' parents to help their children make flash cards on the various words and word families. Send home a worksheet with the different variant vowels you're asking the students to learn and have the children make flash cards out of them. At first, separate the sounds by "sound family" (words that make the same sound). As children progress, go over flash cards with different word families mixed together.
Group the Sounds
As children become more advanced and more adept at recognizing patterns, you can have them group the words themselves. Make worksheets for the children, asking them to circle words that fall into the same variant vowel family. You can ultimately ask the children to write down vowels that fall into the same vowel family without a word bank. This can be used for worksheets within the classroom, done as groups or as a quiz to benchmark the children's progress.