Для составления письма в правильном порядке, необходимо начать с обращения, затем изложить основное содержание, а в конце поставить прощание и подпись.
Ответ: Dear Sam, I'm writing to tell you about my holiday. I'm camping at the lake. It's so enjoyable! I swim every day and fish early in the morning. I think I'll try jet skiing tomorrow. It will be exciting. Thank you for your letter. It was nice to hear from you again. Write about your holiday. Yours, Mike