drafting
After doing your freewriting, you may have one of two typical responses, relief or dread. If you like the freeplay of ideas, you may dread having to give that up and somehow shoehorn your ideas into some kind of structure to meet the requirements of an assignment. But don't despair: if you've given yourself some time, the fun can continue in a "zero draft," a quick discardable version that can help you focus on what's important to you and/or your audience. Also, if you have time and are into the topic, you can use it as an "excuse" to read up and talk about the topic (even with your instructor or some other expert; frosh in the Core course have routinely gotten to speak with mayors of large cities or gone out on field studies just by asking).
You can also continue the exploration by doing what Donald Murray calls discovery drafts: what do I or someone else need to learn about this? Freewrite a draft(s) and then look back at what surprises you. What ideas or images pop up, especially if you do several drafts? Murray says that most people are wrong when they think great writing begins with an abstraction; rather it's really a line or an image, especially if there's a question or contradiction lurking in it.
Also, consider that you might have more freedom than you suspect; I've almost never had an idea for a paper flatly turned down by an instructor, even some pretty wild ones like the paper that was written on a Mobieus strip that seemed to be about the 20th century but then had to be reread when the reader realized at the 'end" it was really about the fifteenth. Worst case scenario, you'll come away from the discussion with a clearer understanding of the assignment. Morover, consider that form is not necessarily the enemy of creativity: how many new ideas and fresh images have been created because a really restrictive rhyme scheme in a sonnet forced the poet to find a word that fit? Consider also the poor filmmakers from Iron Curtain countries who had brilliantly learned to evade censorship who were paralyzed for a long time after the Berlin Wall fell.
Also, thinking about purpose and audience is essential; why bother to generate all those great insights if no one else can benefit from them? Hacker's WR C1-b has some good generative questions on purpose and audience.
If, on the other hand, you're relieved that that prewrite nonsense is out of the way, and now you can get down to some serious work, beware. If you spend a long time staring at the Roman numeral I on your outline, maybe you haven't played with the prewrite enough. Of if you spend LESS time staring than usual, maybe the prewrite helped? If you can't come up with something to say about your topic that most of us don't already know, for sure you need to play with the prewriting more. One of my writing teachers, Andrea Lunsford, advised to keep expectations of a draft reasonable. Any number of writers have expressed the sentiment that all first drafts are [insert scatalogical term here; they've all been used by someone].
It's ok to skip the introduction and just start with the part you're most ready or eager to write about, and then the next because it's easy to move chunks around on a computer. Also be reasonable about how much, but commit yourself to a page or two before getting up, and don't stop until you've left yourself a note about where to go next. Reward yourself when you keep your commitment to yourself.
The Hacker handbook has some useful tips about having at least a tentative focus and organizational scheme, and page 14 has half a dozen good ideas for introductions, which are notoriously hard.