Proofreading your work is the final stage of the writing process and can be key to achieving a good grade. If you'd like guidance on your grammar and punctuation and developing good proofreading strategies:
The Centre for Academic English and Skills is not a proofreading service. Tutors can only spend up to 30 minutes for each tutorial. They are not subject specialists and can therefore not advise on the actual subject-content (or grade your work). It is your responsibility to proofread your own work.
These resources will also be helpful:
Tools to help with writing
Powerful proofreading tips by Debra Smith (ebook)