Teach at whatever rate the student(s) can handle. In some cases, often with otherwise uneducated students, this means teaching only two or three letters at a time. With more motivated students, or with students who have other forms of education, the learning is likely to go very quickly.
Begin in alphabetical order. Write the shape of the letter and then make the sound. Tell your students to repeat, first drawing the shape, then making the sound. If they do not know the alphabet because they are learning a English as a second language, tell them to translate the sound into their own alphabet beside the English character.
Use the alphabet song. It's a proven fact that songs and rhymes help memory. Using the song may sound child-like but if your students are clear on the benefits, it can be a useful tool during alphabet education.
Write down the alphabet with gaps of missing letters in several different places. Tell your students to fill in the gaps as best as they are able. This helps reinforce alphabetical order.
Ask around the room and tell each student to say the next letter in the alphabet. This is good practice because it forces the student to listen to the letter that came before and then think quickly to supply the next.
Practice alphabetizing with different objects or words, placing them in the correct alphabetical order. Spelling contests can also be useful tools for adults to learn the letters.