Choose a blank card or note. While some decoration on the outside or around the border is appropriate, it's more personal to get a card or note that's blank on the inside. Write the letter out in your own handwriting to provide a more personal touch.
Address the letter to your teacher using her appropriate title. For instance, if you're writing a "thank you" note to Mrs. Green, be sure to address her as such. Don't call her by her first name or address the letter to Ms. Green.
Give specific examples of what you're grateful for. For instance, don't simply say "Thanks for the great year." Instead, list particular memories that you're fond of, as well as examples of activities or lessons that you enjoyed.
Write about how you've grown throughout the year. The best "thank you" a teacher can receive is knowing how he has impacted a child's life. Give examples of things you learned that you believe will help you in the future. Remember to be specific.
Thank the teacher once more before signing off your letter. This can be done in one line. For instance, you could say "Thanks again for providing such a stupendous classroom environment for me." You can also say "I truly appreciate all you've done for me this year. Thank you." Use a sign off such as "Warm Regards," "Your Truly," "Best Wishes" or "Sincerely," followed by your name.