Train for a marathon.
Whatever it is we want to do, consistency is critical. We humans really like repetition. We get better at things when we do them over and over. Consistency plows pathways in our brain. With repeated practice a mental game trail of experience becomes an eight lane highway of competence.
All the writing advice for beginning writers. All of it. Everywhere. From all the hundreds of people who have given out this sort of advice. It all tells you to write every day. Some say to journal. Some say to have "junk" pages. Some tell you to get a specific word count, or page count, or amount of time. But all of them tell you to keep doing it.
Keep doing it.
Keep writing. Every day write something. In truth, it doesn't matter what so much as it matters that you show up. Just show up.
Use Wordslingr's Daily Writing Goals to help keep you on track. With consistency comes everything else.