Get into a rhythm by warming up beforehand. Write down any ideas and thoughts that you may have in relation to the questions. A few minutes of pre-writing gives you focus and direction as to how you will approach the questions and will enable you to stay calm. It will also improve the flow and clarity of your essay.
Start with writing a draft in your own understanding of the questions. Give an introduction and a few sentences that relate directly to the question to get a direction on how to approach the other sections. To save on time, just write one draft. Continuous writing leads to exhaustion; it is thus necessary to take intermittent breaks and ensure that you follow directions and not go off topic. Reread your answers to assure that you are addressing what has been asked.
Go over your essay after you have finished writing to correct misspelled words, punctuation errors and structure sentences. Overwritten essays contain such errors due to distractions and exhaustion: proofreading ensures that your essay is well structured and has answered the question.