First, this is how you identify which two drafts you must mark. You yourself have group number (find it on the course webpage). Within that group, you are either the first group member or the second group member, this determined by alphabetical order of your last name. The first group member marks the milestones for the two groups just before his group; the second group member marks the milestones for the two groups just after his group. The interpretation of before and after are numerical ordering with the natural wrap-around, depending on how many groups there are in your section of the class.
Here is an example. Group 102 has group members Matt Franklin and Chip Martel. They themselves submitted a draft d102.pdf. Matt is the first group member and Chip is the second. If there are 12 groups in the class, these numbered 101-112, then Matt grades drafts d101.pdf and d112.pdf while Chip grades drafts d103.pdf and d104.pdf. Matt will upload marked-up files d101-MattF.pdf and d112-MattF.pdf. You must follow the indicated naming convention.
What do you put in your marked-up file? Detailed and useful comments. Please correct both low-level matters and make ample high-level comments. Make your feedback useful. You are trying to help this other group get a favorable reading from the professor.
How do you create the marked-up file? Any manner you like. You can use any tool of your choice, or you can print/mark-up/scan. Adobe Reader XI is free and has markup capabilities: you can fill the draft with sticky notes to your heart’s content.
If everybody does as requested, each group will receive four sets of comments. Note, however, that if the draft you are supposed to review is late, then you are under no obligation to review it: just upload a pdf file that says: “draft late” and you will get full credit.