Hello. You have not informed the type of letter or the subject that the letter will deal with. This makes it impossible for me to answer your question. However, to help you out I'm going to tell you the types of structures you can use in your letter. Hope this is helpful for you.
First, it is important that you know that structure is how the information in your letter will be presented. This structure can be of the Chronological type, if in the letter, you are talking about an order of events that happened in a sequence of different dates or times. The structure can also be of the Comparison/Contrast type, if the letter is shown the similarities and differences between elements. The letter may also present the Problem-Solution structure, if it is showing how certain problems and ways to solve them, or how those problems are solved.
In addition, the card may have the structure called Cause and Effect, which occurs when the letter features certain elements that exist because they were caused by other elements. Finally, the letter can have a structure called Descriptive, which occurs when the letter is describing something or someone.
