It's the best advice one can give to any writer, and in my opinion, the hardest advice for any writer to follow. Just do it. Write. Sit yourself down at a table or at a computer desk or put your tablet of paper down on the nearest giant tortoise shell and write. Make words flow out of your pen and make the words into sentences, which become paragraphs which become chapters which become a complete manuscript. Hey, presto.
The first trick is finding time; because (for me, at least) I can always find something else to do. There are days when the prospect of cleaning out my refrigerator is a more delightful idea than sitting down and writing. Non-writers cannot understand this. "How lovely," they say, "you can spend your days writing. How creative! How fulfilling!" I don't want to shatter their dreams and say that some days it's just "How painful!" I can tell if my writing is going to go well because there's an urge to get my fingers dancing across the keyboard before the words and phrases that are sparkling in my head dissolve into the murky back recesses of my consciousness. I know I'm going to get the next unit done and I may even go further. It's possible for me to write for a few hours before my pace starts to splutter and my fingers become too busy typing typos.
Writers come in all guises, but all of us have responsibilities outside of our writing. It's very easy to allow those responsibilities to take first place and say "I've no time to write today. I've got to get whatever done." It's a simple shift of priorities that affects no one but ourselves. Of course we'll meet our deadline, but we're simply not writing today. We'll write tomorrow. All's well with the world.
There are those writers (how I wish I were one) who will write on and on and not particularly care about the quality of the writing, it's getting to the end of the manuscript that counts. After all, they rationalize, that's what self-editing is for. That's what second, third and fourth drafts are for. Just get to the end and then the real work will begin. I simply can't work this way; trust me, I've tried. If I'm not liking what I'm writing, if the rhythm is wrong or the vocabulary is stale I simply cannot keep going. I have to fix it. I have to figure out what's wrong. For me, there is no point in writing just to write because I know I'll be deleting it before the day is through. I am a very harsh judge of my own work. I like it when my writing makes me laugh, I like it when my writing makes me cry. I do not like it when my writing makes me angry.
Just do it. Sit down and write.