Anne Lamott’s “Shitty First Drafts” is a very interesting and helpful piece.  I found in it many ideas I myself have though at different points, such as the importance of just getting something, anything, down on paper.  I think a big reason many students feel pressured writing is that they rarely write multiple drafts.  So when they’re writing the opening paragraph, it’s not a “shitty first draft” that no one is going to see.  It’s the finished product that’s going to be graded and torn apart.  The anticipation of this sort of criticism leads many students to not be able to get anything down on paper at all.  Nothing sounds right and they don’t yet know what they’re trying to say.
The best way to write a “shitty first draft” is to put down on paper whatever comes to mind, in a sort of stream-of-consciousness writing.  Even if the sentences don’t logically follow one another and the phrasing is awkward and imprecise, just put it down and forget about it.  The important thing is not what your writing, but that you’re writing.  As long as you have something down on paper, you can begin to organize your thoughts and see more clearly the specific topic you want to write about and the points you’d like to make.