Most good books on writing should contain a chapter on editing.
I think the best way to approach editing is to take a text written by the students; tell students to re-read it first of all for ideas, content and organisation of ideas. Students can look at how paragraphs link to each other as well as the organisation within paragraphs. Then they can start working on language. I suggest you break this down; ask students to read through and check tenses and verb forms. Then to check spelling, punctuation, etc. It's important that they don't try to correct all aspects in one reading.
Students can edit their own work or their classmates' work; they can do it individually or in pairs. It is also very good if they can do this editing on a computer.