First off, let me acknowledge that most any business textbook or other traditional source of business info will tell you that the first thing you should do even BEFORE you start a business is to write up a business plan. And that can be good advice.
But for most folks–and I think especially for a consulting business–a full-fledged business plan is a waste of time. Business plans are typically laden with details, projections (which are all pure fiction anyway), and research, which can all be hugely time-consuming. It’s possible to spend months on a plan, staying stuck in the research and planning phase without ever having tested any of your ideas. And that’s all wasted time.
You want to get moving, try things out, see what works & what doesn’t, and get some results. Fast.
Keep it short & simple
That’s not to say you should go off willy-nilly when starting a business. Instead, a back-of-the-envelope plan is good enough. Figure out what your primary deliverable will be, the bare basics that you need to fulfill that deliverable, and how you’ll get your first client. You can figure out the rest of the details (such as your rate, what accounting software to use, etc.) as you go along.
Whatever you’re working on, try to pare it down to the essential tasks that will move you forward. After you’ve achieved some small successes and proven that you can get paid for your skills, you might decide to do some more detailed planning. But at the beginning, you want to focus on moving forward.
Personally, I’ve never had a business plan. However, I’ve had lots of ideas for how I can grow my business, and some general ideas of revenue goals. Beyond that, I haven’t seen a need for a formal business plan.
And don’t bother with financial projections; financial projections are typically used for getting loans or outside funding, and you don’t need or want that. Instead, start your consulting business as a side hustle, a part-time job.