For me it was advice from Dan Harmon: “Don’t try to prove you’re a good writer, you’ll never write anything. Try to prove you’re a bad writer and you’ll write everything.” Not perfect advice but it really does help me write when I’m being overly critical of my ideas.
To be consistent, sit down and write something, anything, every single day.
Even on the days you don’t want to. Hell, especially on the days you don’t want to.
Do it for long enough and you’ll have written your novel, screenplay or story.