Time alone doesn't define a productive focus session. A visible deliverable does. Decide what should exist at the end, prepare only what that result needs, and judge the session by the artifact rather than the minutes spent at a desk.
Choose something you can point to
Work on the project is a direction, but it does not describe a finish. Draft the introduction, organize the reference images, or write three questions for the meeting gives you a visible result. Choose one deliverable that fits the time and energy you actually have.
Keep the deliverable small enough that starting does not require solving the entire project first. A rough version can be a useful outcome if you define it honestly as a draft.
Prepare the immediate workspace
Open the files you need and move unrelated tasks out of the way. Write down any distracting thought you need to remember later rather than following it immediately. The note gives you a place to return without requiring an instant decision.
If the task depends on information you do not have, identify that before committing the session. You may need to change the deliverable from finish the design to list the unanswered requirements.
Review the result, not just the time
At the end, compare the work with your starting sentence. What exists now that did not exist before? If the deliverable is incomplete, note the specific part that remains.
This approach does not require every session to end perfectly. It makes partial progress easier to see and helps the next session begin with a concrete action instead of another broad intention to be productive.
Write down what you want to have in front of you when the session ends: a draft section, a checked calculation, or a list of decisions ready for review. Open the materials that result needs and begin with the first action that changes the work itself.
If you cannot identify that starting action, use the method for breaking a large task into a first step. Once the step is clear, return to your session plan and set a stopping point based on the deliverable you chose.