First I lay out the sections of an assignment—introduction, problem statement, methodology, literature review, findings, conclusion—whatever the instructions call for. Then I start Googling.
I haven't been to a library once since I started doing this job. Amazon is quite generous about free samples. If I can find a single page from a particular text, I can cobble that into a report, deducing what I don't know from customer reviews and publisher blurbs. Google Scholar is a great source for material, providing the abstract of nearly any journal article. And of course, there's Wikipedia, which is often my first stop when dealing with unfamiliar subjects. Naturally one must verify such material elsewhere, but I've taken hundreds of crash courses this way.
After I've gathered my sources, I pull out usable quotes, cite them, and distribute them among the sections of the assignment. Over the years, I've refined ways of stretching papers. I can write a four-word sentence in 40 words. Just give me one phrase of quotable text, and I'll produce two pages of ponderous explanation. I can say in 10 pages what most normal people could say in a paragraph.
I've also got a mental library of stock academic phrases: "A close consideration of the events which occurred in ____ during the ____ demonstrate that ____ had entered into a phase of widespread cultural, social, and economic change that would define ____ for decades to come." Fill in the blanks using words provided by the professor in the assignment's instructions.