You have to write three essays in Section B: one in Unit 1 (writing to inform, explain, describe) and two in Unit 2 (writing to analyse, review, comment; writing to argue, persuade, advise). There are some general rules for approaching these: be aware of audience, format, development/ structure. Also read the question very carefully: it's amazing how easy it is to miss the point, e.g. if it asks about local holidays then Iceland won't do.
Check audience - e.g. fellow students, parents, teachers, parents and students together, members of the public (they may identify a particular group or you may need to select one - if so, make clear in your speech what group you've chosen), head of the school
Check format - e.g speech, letter (formal or informal), article (magazine/ newsletter/ website), leaflet
Note for letters: check the rubric (instructions) carefully - you rarely need to provide an address for letters in exams: the question frequently says 'Start your letter Dear Students' or similar. You do need to have the appropriate ending: Dear Sir ... Yours faithfully or Dear Miss Avery ... Yours sincerely. Remember that signatures do not include your title (Miss/ Professor/ Duke etc) and that you should not waste time designing tear-off slips etc.
Note for leaflets: again check rubric - generally you are asked for 'the words of a leaflet' so do not lay out your information in leaflet style. You may wish to have sections with separate headings and you may have some bullet points but the substantial part of your writing (at least 3/4 of it) should be continuous prose.
Development/ structure: have 3-4 strong points, each of which will make a separate paragraph. Consider whether you need examples, quotations, (brief) anecdotes. You can make up examples and quotations, but keep them short and credible.
When planning and writing, try to keep the audience and situation firmly in your mind: imagine giving a speech to the rest of the form/ writing a letter to Miss Avery etc. Think of how to start to get their attention (but make sure it's appropriate to your audience); consider what style is best (remember, though, that you are demonstrating to your examiner that you can handle language well ... a series of slang expressions and in-jokes is not going to impress him/her). Think of a good ending, making especially sure that you have fulfilled the requirements of the question. Humour can be effective but a sarcastic note throughout rarely works and avoid going over the top in descriptions.