Begin by starting to write.

Sometimes when I was starting a new story and I could not get it going, I would sit in front of the fire and squeeze the peel of the little oranges into the edge of the flame and watch the sputter of blue that they made. I would stand and look out over the roofs of Paris and think, "All you have to do is write one true sentence. Write the truest sentence that you know." So finally I would write one true sentence and then go on from there.

I learned not to think about anything that I was writing from the time I stopped writing until I started again the next day.

- Ernest Hemingway