Teach young children the alphabet song. This is the sing-song arrangement to "Twinkle Twinkle Little Star" that recites the alphabet in order. Even before children realize what they are learning, they will have the specific order of letters ingrained in their minds and will be able to recite them. Don't worry if children don't seem to realize they are saying different letters, or if they run them all together. These students will still be memorizing the concept, and will understand it better later.
Display a board or decoration that has the 26 letters of the alphabet in the correct order in your classroom. Before students even know what the letters mean, they should be familiar with the way that they look and the different shapes they make. Make this a permanent decoration. You can also make a small letter chart for each student and have it taped to their personal desk, so they can look at it each day.
Study each letter, one at a time, beginning in preschool. Introduce the letter, teach the students how to say it, and teach them to recognize the sound it makes. List some words that start with that letter. It is best to start with small words, such as "cat" to teach students the letter C, or "dog" to teach students the letter D. The words should be relatable to the students, so don't use words that don't have a meaning for them.
Study several letters, and then begin putting the letters you have studied together into word groups. After you have studied C, A and T, you can show students how those three letters and their sounds work together to form the word "cat." The more letters you study, the more words you can make.