With shorter papers, it is reasonable to do some of your research prior to writing and look up the rest as you construct your paper. With a long essay, it is crucial to research the information you are want to use in advance so new information does not change the trajectory of your paper in a way that would have a large impact.
Make sure you are making a coherent argument. You will need to be certain your research supports your argument and the argument you are trying to make is logical and cogent. You do not want to begin writing until you are certain you know your thesis statement -- and that your thesis is a logically sound argument.
Lay the paper out in its entirety before beginning. It can be easy to feel lost when writing a big paper, but an outline is like a roadmap you can follow all the way to the end. Include important parts of your argument and supporting quotations in your outline to make writing the actual essay much easier for yourself.
Break the paper up into sections, and write them one at a time. Writing a long essay in one sitting leads to incoherence, as you can lose your sharpness after a few continuous hours of writing. Take breaks, give your mind a rest and write the paper in small bits over an appropriate span of time.
It is hard to proofread a long essay, especially after you have spent so much time looking at the same piece of work. For your final proofread, print the paper out and read it aloud to yourself, making sure everything you have written makes sense and flows well.