I always type a letter with the following at the upper right hand corner:
Applicant: (Full name here)
UCI: ------
File number: --------
Then I type the following:
Dear Immigration officer:
I am attaching to this letter the following additional documents as requested:
(Description of document here like, PGR police check)
I appreciate your time and I look forward hear from you soon regarding the status of my application for permanent residence.
Sincerely,
(applicant's full name and signature)
It has worked pretty well so far