If you haven’t written for a while, the first thing to do is consider your life today. How much time do you have for writing and when during the day or night would be the best time to write? This can be important because if you don’t schedule writing for when you have the time, you’ll eventually quit because it won’t fit your life
During that perfect (or as perfect as you can make it) time, do something writing related. Start a journal and keep it going. Scribble random thoughts on a piece of paper or a computer. Let your imagination go wild and picture all kinds of stories and characters. Whatever works for you, so long as it is somehow related to writing.
As time passes, you’ll notice that you spend more and more of that ‘writing’ time doing one particular thing. Concentrate on that one thing and focus on doing it as well as possible.
Learn the craft of whatever it is you wish to write. Take lessons, read books, whatever.
Use that one thing you now do well and rework it according to the rules of the craft of writing you’ve just learned. And you’ll discover that you have written an article, short story, poem, or book.