Choose the major common points in the evaluations that you would like to address in the memos. For example, if a number of instructors are receiving low points for organization in their lectures, these points should be discussed.
You should also create a chart with a series of curves to illustrate the highs, lows and averages that instructors in the department received on various categories, keeping names and other identifying elements out of the memo, of course.
Draft your memo based on the information you gleaned from the evaluations and add your chart. Since you don't want the memo to be completely negative, make sure that in addition to any problem areas you point out, you also mention the overall good job being done and the improvements being made in many areas (if any). Send this memo privately to instructors.