Don’t exaggerate -- show where and how they can improve. Don’t put the candidate on a pedestal. Not only does it not look plausible, it will also set expectations for them that will be next to impossible to meet. If they have an Achille’s heel, don’t exaggerate it, but don’t hide it, either.
Use standard formal letter writing conventions to begin. A letter of recommendation is like any other professional communication, and follows the same general rules and guidelines. Place your address on the top right, followed by the date—spelled out. Below that, on the left, place the recipient’s name (if known) and address. Open the letter with a formal business greeting. Ex: Dear Ms. Smith, Dear Sir or Madam, (if you don’t know the recipient’s name)