The traditional way for children to learn the alphabet is through singing the "ABC song." Trying to do this with adults will cause you, and them, to feel a bit silly. While many foreign languages also make use of the Roman alphabet, many others do not. Learning the alphabet and the phonics sounds that go along with it is an essential skill for those who wish to master the English language. Use grown-up methods to teach the alphabet.
Pass out writing worksheets to do in class. Your students can learn the proper way to form the letters in writing and the repetitive motion can help to remember the letter. Each sheet should have a single letter. While passing out the sheets, say both the name of the letter and the sound the letter makes.
Use flash cards to drill the letters. Show each letter and say its name or sound and have the students repeat. After you've gone through the deck once, go through it again, asking the students to say the letter name or sound without your prompt.
Pass mirrors out to the students. Certain sounds do not exist in other languages, which can lead to problems in speaking the language. Show a letter while making its sound -- exaggerate this so that the students can see the position of your lips, teeth and tongue. Ask students to use the mirrors to make sure their mouths are forming the sound correctly.
Combine letters the students know to form words. When you start to put the letters together, they become more meaningful for the students.
Use books to encourage letter recognition. Take note to use materials designed for ESL adults, as using simple children's books may be insulting to students.
Don't expect your students to pick up the alphabet in a day. It should take several lessons before students will start to know the letters.