A good extended responses has: good case studies that are comprehensive, good illustrative examples, and good structure.
Having one or two really detailed case studies that cover all the syllabus dot points is far better than having several to cover all of them, as that way you cut down on how many stats you need to memorise (more case studies=more stats in total cause they'll be all different), and it will sound like a more comprehensive answer. For example, in population geography, you could have one case study that has examples of environmental, economic and social impacts from changing size and distribution of population, which may arise from the factors explained in the previous dot point, that is, rural-urban migration, labour migration etc. In year 12 when you're doing urban places, it's quite handy to have one case study that can apply to both health, education and water challenges in mega cities, and one response that covers all three areas at once. Of course, finding these examples, if your teacher hasn't found one for you, will take time, and often info won't be readily available, but if you can, it's preferable. Having examples that can apply to multiple factors also allows for a better flow between paragraphs.
Including at least one Illustrative example is essential, as its in the marking criteria. In practice, these are quite easy, if you can think of a good one. I often did one to explain a concept, eg. The aspect and slope of vineyards and why that occurs, or a problem, eg. A flow chart showing the demise of the small town, or a management strategy, eg. Planting roses to fix nitrogen in vineyard soils, or illustrating stats in a nice way, eg. A pictorial showing where Mexico City gets its water from (fun fact it's sinking!). But make sure your illustrative example makes sense, and it's not just there to satisfy the marking criteria. I'm not sure if it's actually in the marking criteria, but if there's something in the stimulus booklet that relates to your case study/idea, then defintely reference it.
Lastly, structure, like any typical English essay, is important. You know, listing your arguments, having a thesis etc.
Make sure you look at the directive term- discuss, analyse, evaluate, assess, etc. The more extended responses, or even just body paragraphs that you write, the more you'll get the hang of writing them. I liked planning out my responses before I started them, which is favourable, but I know some people don't, and they get away with it
Hope this (ridulously long) advice helped you
