The first, because it is largest, most evident, and the most mechanical (which means the easiest to think about and implement) is your environment.
The pervasive image of the starving artist huddled, shivering in their garret leads us, perhaps unconsciously, to believe that art is immune to environment, or even that art is created by pain and suffering.
Your rational brain knows that this is nonsense. The world’s leading expert on creative flow agrees:
A proper environment, as Csikszentmihalyi points out, adds enormously to our ability to create.
Here are a few things you should carefully inspect to ensure that they are the best you can arrange for your writing environment. Some will have a large effect. Some will have a small effect. But all will affect the comfort and ease of your creative abilities.