The Basics: Handwriting, Spelling, Grammar, etc.
You may find the advice contained in this and the following section (on Some common mistakes corrected) rather patronising or unnecessary. Perhaps, for you, it is unnecessary. But then again, perhaps it isn't. All the advice contained in these sections is simply accumulated from comments that I have regularly needed to make when marking undergraduates' essays in Cambridge.
Examiners are only human; maybe they should not give very much weight to things such as handwriting and spelling, but I can tell you from experience that marking an essay that is difficult to read because of the handwriting, or which contains a lot of basic errors of spelling or grammar that need correcting, is a wearisome and unpleasant experience. Wearisome and unpleasant experiences put people in bad moods. And people in bad moods give essays lower marks than people in good moods do.
You can do yourself a big favour by making sure that you have learned to write legibly, spell correctly and use grammar properly by the time you take your exams. Your aim should be to make no spelling or grammatical mistakes in essays for supervisions or in exams: Every such mistake is unnecessary and can be eliminated by a careful read-through of your essay. (Although, being realistic, we all miss some mistakes even after reading a piece of work through several times, and so one or two undetected slips will not count against you).