Your essay will be scored according to four domains: Ideas and Analysis, Development and Support, Organization, and Language Use and Conventions. (Notice that these correspond with the four bulleted items in the Essay Task.) What is significant for you, prep-wise, is to make sure you can write an essay that is well developed in each of these four domain areas in order to maximize your Writing test score.
Two trained readers score your essay on a scale of 1–6 for each of the four Writing test domains; those scores are added to arrive at your four Writing domain scores (each from 2 to 12). You will also receive an overall Writing test score ranging from 2 to 12, which is determined by a rounded average of the four domain scores. Essays can receive a zero if they are entirely off-topic, left blank, illegible, or written in a language other than English. If there’s a difference of more than 1 point between the two readers’ scores (for example, one reader gives the essay a 3 and the other a 5), your essay will be read by a third reader.
Statistically speaking, there will be few essays that score 12 out of 12 for all four Writing test domains. If each grader gives your essay a 4 or 5 for each of the four domains (making your subscores 8–10), that will place you within the upper range of those taking the exam.