To write, you have to feel comfortable and allow yourself authority. (You are also best not suffering sharp pain, and your mind must be clear.)
It helps also if it is what you want to do. I wanted to do it for a long time or was happy to do it.
For a long time I refused to do it because it was part of what I had been ordered to do. I had been ordered to remain in academia when it was not what I wanted, at least not in the circumstances I could attain, and when I had lost interest in what I was writing and developed a burning interest in something else. I had to stay in academia to atone, it seemed, for the sin of having gone to graduate school, and because people felt I owed something to the field. People were also afraid of change and did not want me to do it, and disapproved of what I was planning to do because they did not understand it and were not interested in understanding it, but only in pontificating.
So I had been too torn down by Reeducation to write, and was too torn down by my alleged supporters to make any other move, so I was lying on the floor with all paths blocked. Do you think this was a reasonable situation to have? I am just amazed I got as much done as I did. It was a ridiculous circumstance with a lot of dead time and also wasted effort, but I am still amazed at all that was done.
To write, you have to feel comfortable and allow yourself authority. People do not appear to recognize this, but I note it.