I just saw your message. I wasn't the greatest at SDD. I actually found myself struggling in some areas. But seriously, the best advice I can give you is to make sure you know every single dot point in the syllabus but more importantly, under which chapter/heading and topic it comes under. I cannot stress that enough, it's something I wish I had done. The questions generally make mention of a particular chapter. If you know where everything is then you'd know precisely what to write about without waffling on.