Organizing your refusal letter before you begin to write it will establish a logical order in which to present your information. You have already begun this task by establishing an objective and determining your scope. Refer back to them. Together they include much of the content that will become the body of your letter.
A simple outline will get you organized. Begin by creating a list of relevant points and place them in the sequential order that will best help your reader comprehend your refusal. These points will become the backbone of your working draft.