Decide on your thesis, the topic of each paragraph, and the arguments which you intend to cover, then jot down some quick point-form notes. This process won`t take long, and, once you complete it, all that`s left will be to expand those notes into a well-organized essay.
However, it`s important to keep in mind that your professor understands the circumstances under which the essay was written. They`re fully aware of the time pressure you were dealing with, and they will judge your work far differently than they would judge a typical essay with a deadline set weeks after the assignment date.
If you did your best to write a complete, clear, and insightful essay within the time allotted, you should have nothing to worry about.
Although the conclusion paragraph comes at the end of your essay it should not be seen as an afterthought. As the final paragraph is represents your last chance to make your case and, as such, should follow an extremely rigid format.
Take note of the amount of time allotted for the exam and split it into reasonably-sized segments, leaving some time at the end for revision if possible. Without a schedule to follow, it`s easy to become too focused on a single paragraph and run out of time to finish the essay.
For this reason, it can be helpful to simulate the conditions of a timed exam before the actual day: pick a practice question, find some lined paper, set a stopwatch, and see how you do!