I don't know your English level, but write in English anyway. This format is similar to letters in our country. In terms of content, you can introduce yourself first. For example, you are a real Madrid fan in China and love Real Madrid very much. Then you can write some history of Real Madrid, such as its brilliant achievements in the past 100 years. Then you can talk about the recent situation of Real Madrid, such as record and personnel changes. . . Finally, you can express your love for Real Madrid and hope to get some small gifts (if you are lucky, you can get a star card sent back by the club). And the writing of the envelope is different from that in China. . . You can buy a magazine of the football club. At the end of the magazine, there will be a section for fans to communicate. Fans will show their club letters and small gifts, and you can communicate with these fans. . .