Sometimes, just getting started is the hardest part of writing something that's in your head.
I just read a blog about this, and I'll post one paragraph, which I thought was great, here, and you can follow this link to Scott Berkun's blog to read the rest.
"Writing is easy, it’s quality that’s hard. Any idiot who knows 5 words can write a sentence (e.g. “Dufus big much Scott is”). It might be grammarless, broken, or inaccurate but it is writing. This means that when people can’t start they’re imagining the precision of the end, all polished and brilliant, a vision that makes the ugly clumsy junkyard that all beginnings are, impossible to accept. Good voice, tone, rhythm, ideas and grammar are essential to good writing, but they’re never introduced all at once. I promise you, the first draft of Strunk and White didn’t follow Strunk and White. The secret, if you can’t start, is to begin without constraints. Deliberately write badly, but write."
