I always consider cash flow as the most important model for the business. Even if your new business relies on the production of a product before selling, project planning and planning must be accurate (or your cash flow is a disaster).
Many people think the cash flow (and the business plan) is static but not so.
My advice is to use a spreadsheet to create a business model.
Define the variables and try out different scenarios by changing them. You may want to finance your business plan, but it will be of great benefit to you.
The hardest part of a startup is to achieve an accurate cash flow. This is especially true if you are doing something new and innovative. That brings the experience partly into a company.
When you create a "model" in a table, you can test best-and worst-case scenarios. It is important that you also receive numbers that you can use to benchmark at startup. This is important so that you can take corrective action early when problems occur.
If you have a trusted contact in a similar company, show them your model and see if it looks realistic. Check with them if your basic indicators such as sales, overhead and gross margin are in order and what is expected for the industry.