Is this half-yearly appraisal or annual appraisal? If it is routine then why would you like to send mail?
Do you have "Policy on Performance Appraisal"? If yes, then all that you need to do is send a mail to all the managers to follow the policy guidelines as per the policy. If you do not have policy and then you need to create the policy first. Generally, policy guidelines are given on how to conduct performance appraisal, when to conduct, who will conduct whose PA and what are the general pitfalls in the PA process.
While HR department should be as communicative as possible but then it should avoid unwanted communication as well. Otherwise, HRs will get engaged with the unproductive activities and crowding of the unproductive activities in turn could erode their own performance.
Notwithstanding my above suggestions, I recommend you writing draft on your own and uploading on the forum. Some senior like me will correct it. Asking for a ready-made draft is akin to asking for spoon-feeding which obviously is not salubrious for your career.
21st November 2018 From India, Bangalore