This is the approach I take.
Evaluate your business idea honestly: as Catherine Fake, founder of Flickr.com once said: “So often people are working hard at the wrong thing. Working on the right thing is probably more important than working hard on the wrong thing." I’ve got a business model evaluation calculator here.
Decide what kind of document you need: that’s right, there are different kinds of business plans. At one end of the spectrum, there‘s back of a napkin style plans and at the other, full blown prospectuses. The reality is, most entrepreneurs need something in the middle, a fairly simple business plan whose main objective is to guide and document business planning - quite literally, sitting, thinking, researching and deciding.
Start by starting: it's easy to procrastinate on these documents, often because they are unfamiliar. Give yourself permission to be imperfect and learn by doing. A basic table of contents of a solid BP includes: executive summary, vision, mission & values, management & operations, services & products, market analysis, marketing & sales plan, financial projections, exit strategy, appendix. Use bullet points if you have to!
Involve outside help if you need: good finance analysts will know writing business plans like the back of their hand. My own process starts with a robust, 42-part questionnaire, then I sit down with clients and piece together a document jointly, to demystify the process.
Treat it as a working document: in an ideal world, you would literally be opening your business plan each day to help keep you on track, but also be tweaking and refining as you go along. That's often practically unrealistic, but you should still isolate some time each month or two in your calendar to review and update the document.