What is written with ease is never read with pleasure.
-- Samuel Johnson
If you are used to whipping off papers the night before they're due, running them quickly through the computer's Spellchecker, handing them in full of high-school errors and sentences that make no sense and having the professor accept them 'because the ideas are good' or something, please be informed that I draw no distinction between the quality of one's ideas and the quality of those ideas' verbal expression, and I will not accept sloppy, rough-draftish, or semiliterate college writing. Again, I am absolutely not kidding.
-- David Foster Wallace
For mathematical answers you should know how to write mathematics. For that, you can see the Math Preliminaries reference below.
The examples of 173 project writeups are not bad and give you the idea, but you can always improve on them (especially with references!)