Yep, I do the same as Rick & Shani described; Initially just start typing a list of questions (What, how, who) under my section or chapter heading, then start answering those questions. While answering those questions I get many ideas and remember many other concepts, important statements and start writing in pointers; then organize and expand those pointers, answers. Main key to start a structure is, I think, to start writing what you want to, your ideas and not what your advisor/supervisor expectations. Once the structure is formed then work on those expectations.