Editing, of course, is a very complex activity. There is so much to look for. You might want to consider focusing on only one item at first and build up to more. You can even assign different responsibilities to each group member. Rotate the roles periodically.
Checklists are especially good for editing.
This could be a pre-prepared list or a list posted in the room and added to as the year progresses or a list in each student's folder. You could even have a continuous checklist for the whole year that shows a record of each edited piece. During both revising and editing, the issue of loss of ownership can be a problem. This is particularly true if you or students mark on the papers. Use pencil for marking so the author can readily erase marks As part of revising and editing, have students experiment with a variety of revising and editing strategies.