Here’s a neat post from Sally Kerrigan, where she helps writers to make that sometimes terrifying rough draft:
Now you just need to start putting your ideas on paper. Try not to reread until you absolutely have to, preferably on a different day altogether. Just think about what you’re trying to say, and jot the main ideas down. If you’re not sure how to finish a sentence, abandon it halfway through. If you want to write extensively about one particular idea but your mind’s moving too quickly to flesh it all out, paraphrase for now and move on to the next big point.