Stop waiting for inspiration or motivation and do it. Writing is hard work. Sit yourself down with a piece of paper and start working.
Make yourself work for 15 minutes. You'll probably work for a lot longer than that once you get going.
Get in the habit of writing every single day.
"This is how you do it: You sit down at the keyboard and you put one word after another until it's done. "It's that easy, and that hard." - Neil Gaiman