Decide whether you can write an honest, positive letter for the student requesting the letter. If you can't truthfully recommend him, and can't think of a way to write a letter without talking about his weaknesses, politely decline to do the letter. Never lie in a reference letter.
Talk to the candidate to find out exactly what she wants. Determine if she is looking for a letter to get into school or for a job. Ask if she wants you to focus on her academic achievements, personal achievements, job skills or something else. Request she tell you things she would like you to highlight, and a list of achievements and notable skills.
Determine the format you need to write in. Some companies and schools want only a letter in a sealed envelope, while others have a form you must fill out, or want you to send your letter online.
Write in business letter, or block format, with all text justified left. On top write your address. Then, after a space, write the date. Type another space and the name and address of whomever you are writing the letter to. Put in a space, then the greeting: preferably the person's name, or "To whom it may concern" if you don't have a specific name. Enter the body of your letter after that with a space between each paragraph and a space before ending with "Yours sincerely" or "Yours faithfully." Hit enter four times, then type your name. When you print the letter, sign in the space you left between "Yours sincerely/faithfully" and your name. Look up "business letters" or "block format letters" online if you are still unclear.
Write about the student's relevant strengths. Some strengths, like flexibility, a good attitude and the ability to work well with others are relevant to most reference letters. However, if someone requests you write a reference letter to help him into music school, don't write about how good he is at woodworking. Focus on his skill playing the piano, and what a hard worker he is at music study.