A historian's primary task is to analyze, synthesize, and interpret primary and secondary evidence through writing. Throughout your graduate career in history you will deepen your ability to ask questions about different kinds of historical evidence, critically evaluate that evidence, and craft an argument to persuade your reader of your interpretation of the evidence. You will gain an understanding of the different methodologies and styles of history--social, cultural, political, economic, etc.--and learn to critically evaluate secondary works. All styles of history formulate theses based on interpretation of evidence and persuade the reader using well-structured arguments.
Many of these sites are geared towards undergraduates but offer great information on the nuts and bolts of history writing.
Mary Lynn Ramponella, A Pocket Guide to Writing in History
A wonderful, concise guide that details the purpose of historical writing as well as how to conduct research, craft an argument, and the use of evidence. Written for undergraduates, but highly recommended, as it spells out many things that you might know instinctually as a graduate student in history but will aid your fluency in crafting your own arguments.