Make a game in which the students need to identify diagraphs. Place the students into several teams. Place three words on the board, one of which contains a diagraph. The first student to raise his hand and correctly answer which one is the diagraph gets a point for his team. The rest of the students should identify the diagraph as well so that everyone can practice identifying them. You can have prizes for the teams that get the most points to make it enjoyable for the students. The students will be motivated to recognize the right words and will have fun playing.
The children should identify words in a story or poem that contain a vowel diagraph. The story or poem should contain many words with diagraphs, as well as some words that are close but not the same, to give them a lot of practice. Go through the answers explaining why some are diagraphs and some are not. They can go through the same stories again or give them new pieces to reinforce the concept.
Instead of just asking the students to identify vowel diagraphs, ask them to come up with their own. Instruct them to write as many words with vowel diagraphs as they can. They can write them on index cards so that they can be used as flash cards to study. Have them describe to you in their own words what a vowel diagraph is, either by writing it out or verbalizing it. They can also use the vowel diagraphs to make their own poem or short story. This activity will be more appropriate for later in the first grade year when the students recognize more words and can read better.
Look to the Internet and software for a multitude of computer activities that teach students about vowel diagraphs. Interactive activities include games in which the children need to choose which word on the screen contains the vowel diagraph. Many first graders enjoy working on the computer, thus this can be an enjoyable and efficient way for them to learn.