Use flash cards to introduce the letters of the alphabet. Students won't learn to write the alphabet without some type of context to help them understand the meaning. Show them the letters and make the sound, including a few words that start with that letter. For example, you might say, "D...d...d...dog. Doll. Door."
Pass out writing worksheets to the students. These should include three lines for proper spacing of the letters, a demonstration of how to write the letter, a few dashed practice letters for the students to trace and ample space to practice their own writing. If possible, choose a worksheet that also includes a picture of a word that starts with that letter.
Demonstrate how to write the letter on the board. It can help to say what you're doing as you write. For example, when writing an "A" you might say, "Down. Down. Across."
Allow the students to practice writing their letters.
Walk around the classroom as students work. Offer help to any student who seems to be struggling.
Repeat with new letters each class.
Practice writing without guides, as students begin to master the letters.