Try to start with the easiest portion of your exam. If it's a single memo, write the facts summary section. if it's a multi-part essay test, pick what looks to be the easiest question and write some kind of an answer for it. It doesn't have to be good, you're just priming the pump so to speak. If all else fails, start formatting your outline to look like an essay and, as you do, fill in any of the thoughts/analysis you are considering as you do.
It's SOO much easier to get working when you've got something on paper, even if it's total garbage. Let the garbage flow and clean it up later.