Bluntly speaking, most annual report executive messages are dreadfully boring or irrelevant. On the other hand, the best executive messages are more like summaries of the annual report.
The message should make an emotional connection with the readers by reminding them of the good work you are doing to ultimately make the world a better place. Use the letter to set the tone or theme for what you are sharing in the rest of the report.
If you treat the message like an executive summary, your letter can look something like this, with each line representing one paragraph.
* Overall focus of last year
* Accomplishment #1
* Accomplishment #2
* Accomplishment #3
* New emphasis for the current or coming year
Following the above model, your letter will have five paragraphs. Keep your paragraphs to three to four sentences each, with the total length of the letter no more than 500-600 words