It happens. It happens a lot. You've spent the whole day on other tasks - attending classes, sitting behind a desk, scanning things on a register, paying off loan sharks, etc - with an idea in the back of your mind. It could be an idea you've had for some time, it could be an idea that was sparked during the course of the day. It's been baking in your subconscious, and you've been poking it with the occasional fork as new thoughts modify and expand the idea. By the time you get home you're ready to dive into it and get it on the page.
But nothing happens. You stare at the paper, tapping your pen against the blank page, trying to figure out where to begin. How to begin.
There's a simple reason for this. Your brain is a bastard.
Too often when I sit down to write or to draw, my brain gets in the way. Some might call this writer's block, but this expands beyond that, and affects so much more. When drawing a still-life, the brain will interpret the image that you are seeing incorrectly. You keep looking down at the page, brow crinkling in frustration, wondering why the hell your picture doesn't resemble what is in front of your eyes. Why isn't this scene working out the way it should? It's just as vivid in your mind now as it was when you were mowing the lawn two hours earlier, but now...
The lines look wrong. The dialogue is weak. The colors aren't working. What the hell is wrong with your kerning?
I spent years wondering why this happened, and it wasn't until three semesters ago that I learned the problem. "Your brain is a bastard," my instructor said. "Don't listen to it."
There was an explanation behind it, but I was struck so deeply by that remark that I honestly cannot remember another word he said that day. Ignore what your brain is telling you, and just do it. Your thoughts sometimes get in the way, insisting that things should be a certain way. Insisting that you do it that way, or do it better. Perhaps telling you that this wasn't that good of an idea, or that you just don't have the skill to pull it off.
I find a glass of good scotch shuts that bastard up, at least for a little while. The simpler solution - and considering the price of what I consider to be "good scotch," the much less-expensive solution - is to tell that fucker to shut the hell up, and ignore it.
Whatever you're working on doesn't have to be amazing the moment you begin, and it won't be. Every writer needs to revise. Every artist needs to crank out thumbnails. The important thing is to get the initial idea on page, so that you can see it. Until you can read the words, examine the lines and shapes, the idea is potential and not yet kinetic.
This is not an easy thing to do. We spend our lives trusting the grey matter, and ignoring what it tell us goes against our instincts. I've spent more than a year now attempting this, and I admit that I am only occasionally successful. As with writing, drawing, or any other creative outlet, it takes practice. So kill two birds with one stone, and practice ignoring your brain while you practice your art.
Unless you can afford a nice Glenfiddich.