It is easier to focus when you know the next step. Before you begin, define a small, visible task, such as reading five pages or drafting the first paragraph.
Close what you do not need and keep your materials nearby. Use a timer to set aside a period of focus. If another idea comes up, write it down and return to the task.
Rest matters too. After a work block, stand up or look away from the screen. Notice which session length helps you stay attentive without wearing yourself out.