Writing a thank you letter is something you should start doing if you have not been doing it before. It goes a long way than you can think. However, here are some tips on how to write a thank you letter.
1. Start your letter with a salutation, probably after your address, if needed. Your salutation can take any form, such as "Dearest Jane," "Hi, Jane," "To my favorite Jane," and the likes of you want something more personal. Nevertheless, you can use the common "Dear Jane," for example.
2. Go straight to the point by expressing your sincere gratitude with reference to an action or event. Mention what they did, and let them how it was useful to you and how much you really cherished it. But don't forget to make your writing short.
3. Taking another paragraph, try to share a little news about what matters, or you can choose to make an inquiry about the person.
4. Before you close your letter, write the conclusion part, by appreciating them once more. Then you can conclude your letter, but it's not compulsory you stick with "yours sincerely" here. You can address the person based on the relationship between you.