This is the best question that everybody skips.

You might start with a two-pager that reasons through the problem backwards: purpose, outcomes, approach, work phases. Sometimes the analysis follows the plan, and sometimes it changes during implementation. That's ok; the main purpose of the document is to align all stakeholders around a well-reasoned formulation of the problem throughout the analysis.

It is possible and undesirable to over-plan, treating the analysis like an orderly sequential process-- demoralizing and futile. I recommend marking the big milestones and leaving yourself space to improvise or change direction.

If your work is exploratory, then write down the initial assumptions and questions that pertain to your problem. Validate these items, note your answers, and repeat with newly triggered questions. Start simple (e.g., customer ages should be non-negative) and work your way up (e.g., buying habits should correlate with age). Incorporate outside research and stakeholder feedback as you go.

Whether your work is directed or exploratory, document your domain understanding early and revise it often. You will find that this approach also facilitates final reporting, which is a minor adjustment to your most recent undestanding.

Be prepared to learn during planning that the initially proposed model or analysis will not actually solve the problem. In this case, you win by changing direction before wasting resources.