Developing a habit is not the same as writing content for a book. You’re trying to get to write something, anything.
This is not publishing material, this is writing the homework you missed in school, writing the diary you didn’t care to keep as a child, posting to the blog you’ve been trickle-posting to, it’s putting down in writing your opinion and thoughts on whatever subject interests you.
Writing can be anything from reviewing products to aggregating resources. The ideas don’t have to be new but they have to be presented with your own words.
Slow, consistent progress is the only way to make big things happen. And the ‘big thing’ here is getting into the habit of writing consistently.
Start small, really small. 20 to 50 words a day is enough. The idea is that you should be writing something, anything, everyday.
It does not matter how slow you go as long as you don’t stop.