Write down the skeleton framework "essay outline" and populate it sparsely with just the main points. One to two sentences for each point you're going to make is usually good enough.
When you sit down at the exam, spend the first few minutes rewriting that framework on the back of the exam sheet, and then use what you studied to expand each point's sentence into paragraphs, and go from there.
I've had students hell-bent on actually writing the exam's essay beforehand, stuffing it all into their brain, and then attempting to break the land speed record writing it all down before the words started leaking out of their ears and whatnot. It works for some people, but you probably already know if you're the type of person who can do that.