You've completed your research. You know something about the person to whom you are writing. You have a good idea what topics will be covered in the letter. The information you have gathered must now be analyzed so you can logically organize it for the best results.
An outline is a good method of organizing topics and visualizing the order in which you wish to discuss them in the letter. You can order the letter chronologically, by importance of the topics discussed, or in whatever order is most effective. Your choice is flexible, but it must be logical and you should not mix thoughts in sentences or drop them before they are completed.