Memo 1: Positive Message
Assignment: As the Human Resources Manager at XYZ Industries, Inc., it is your pleasure to announce the addition of annual performance-based bonuses for mid-level managers and above.  These bonuses will require annual performance reviews to discern the amount of the bonus.  Draft a memo explaining this scenario to this specific group of employees.

Preparation: Review Chapter 10: Writing Routine and Positive Messages (Especially pages 266-267).

Format: Memo formatting can be found on page 167-168, 590 in your book. 
The format of your memo tells your employees a lot about your sensitivity and professionalism. Since you do not want to upset any employees unnecessarily, you should also be conscious of your wording when explaining culturally-sensitive subjects.  Pay particular attention to your memos margins, line spacing, font type and size, and placement of memo parts such as the addressees, subject line, and body.
    Address the memo to all mid-level managers and above at your company. 
Recommended Organization

Header:  Begin your memo with the four standard memo heads (Date, To, From, Subject).   

Body/Discussion:  In the first paragraph of the body, start off with the good news.  Memos do not begin with greetings or salutations.  Explain what factors could have contributed to this new development (i.e. increased sales, raising stock price, etc.) and why the decision was made to reward employees with the surplus funds.

The second paragraph is where you explain the new process that the employees can expect.  Outline the way that reviews will be handled, and by whom.  Feel free to be creative (yet somewhat realistic) when outlining how the performance scores will translate into bonuses (i.e. tier system, seniority, contribution value).
In the third paragraph, be sure to validate any anticipated concerns employees may have about this adjustment.  Explain the greater benefit in the long-term this decision provides and conclude your memo by indicating how the reader may contact you if he/she has any questions.  In general, this section should be short & sweet, making sure that the employees know what is expected of them.