What should I do to get full marks in the essay section?

The essay section is split into 3 individual questions. These 3 questions will be worth 20 points altogether, out of which 2 points will be awarded for the quality of your writing, i.e. hand-writing, grammar, spelling, punctuation, neatness, etc.
The 1st of the 3 questions tends to be a question in which they ask you to draw a diagram. You must ensure that (a) all the diagrams that you draw are in pencil, (b) that the lines of the labels are drawn with a ruler, and (c) the lines of the labels come off from the diagram horizontally. The lines of the labels should never cross each other. I would advise you to learn all the diagrams mentioned in the syllabus by heart as these questions very often repeat.
The 2nd and 3rd questions are essay based, but I would advise you to draw a diagram wherever you can if appropriate. Always define the biology terms present in the question. I suggest that you write as much as possible about the relevant aspects of the subject area, even though it may not directly be linked to the question itself. Examiners like seeing extra knowledge on paper, and you never know what that year's mark scheme will award marks for. Of course, questions repeat themselves, and I have all the past papers and the official mark schemes, which you can learn from as well.