Now comes the tricky part--moving from grossly identifying error into setting up a plan to teach students how to edit for those errors. My rule of thumb is to start with the errors that are most confusing or distracting for readers--sentence punctuation, subject-verb agreement, and garbled sentence structure. Especially with native speakers who are inexperienced or basic writers, these are most likely to be the errors that mark them as ineffective writers. Native speakers trying to impress readers with a big vocabulary are most likely to have word-choice and spelling errors, but they may also misuse semicolons; of these, the word-choice and sentence-punctuation errors are the most significant. For non-native speakers, subject-verb agreement, verb tense, and article errors are typically the most noticeable and distracting.
Even though you may have a pet peeve about a particular kind of error, try to put that aside to focus on the errors that most readers will find disruptive of communication. Those are the errors to start with.
Go back through the paper and rank the errors in terms of their disruptiveness. At this point, it's often useful to make lists on a second sheet of paper or to work with a grid that will help you organize the errors you spot.
Don't forget to try this step with your sample paper.