Write naturally for 5-10 minutes and see what you have. Some of it might be rip-out-your-hair terrible, but a lot of writers find their best work out of natural writing.
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AAAAHHHHH!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
I agree. Whenever I just let my fingers do the walking over the keys and do not stop to worry about it being perfect, but concentrate on expressing what's in my heart instead, I usually impress myself! ;D