Start with a quick read-through so you can get an overall idea of content and writing style. Make grammatical corrections. The most important thing is content, so don't spend considerable time on grammar. If you are not sure about something, you can look it up or just leave it alone. Don't guess. If you notice excessive grammatical errors, make a note referring these students to the college's writing resource center. Keep in mind they may be ESL (English as Second Language) students. As you read, begin formulating the major things you want to address in your comments.
Read the papers again and start to make comments in the margins. Evaluate the theses first. If they need strengthening, give suggestions. State if material is too general, if more specific examples are needed to support the theses and if any inaccuracies exist. If doing a rewrite is part of the assignment, refrain from specifying what to do. If you simply offer open-ended suggestions, the authors will have to think it through, improving the opportunity for learning. Be sure to cite instances of something good. Exclamations of good work will bolster student self-image and foster excitement about education.
Write a summation at the end. First, state what was good, then give the questions that remain to be addressed. Specify sections needing further development. Present suggestions for overlooked resources that would have proved helpful. End with some words of encouragement.