Write down on scrap paper the important lessons your instructor has taught you or the specific occurrence you would like to thank him for. Include recommendations he wrote for you or time out of her schedule that she dedicated to helping you.
Start your letter by addressing your professor by name on a blank note card. Always use his title followed by his last name, unless he has invited you to call him something else. Continue by naming the course you took with him during the semester and important lessons you will take away from his lectures.
Write specific examples of how the professor is different from the other instructors you have had. Pick a standout moment to thank her for, such as staying late to review material for the test or writing you a recommendation for an internship.
Finish your letter by explaining how your professor's guidance has changed you for the better. Maybe with his help you landed an entry-level job or maybe he succeeded in making calculus less painful than you ever thought it could be. If it is someone who has had a significant impact on you, pledge to keep in touch.
Sign your letter with "Sincerely" or "Best Wishes," followed by your full name and graduation year. Below your name, include your e-mail address in case your professor wishes to contact you. Seal the card in an envelope.