Good job tackling this important exercise. There are many entrepreneurs who shy away from it.
That said, if you're in the US, check out SCORE (https://www.score.org/). The organization has chapters around the country or you can connect virtually. This is the perfect type of project for their counselors.
If you wish to discuss, send me a PM through Clarity for 15 free minutes.
I like Kerby's answer. I'd also suggest working with a fractional CFO service like B2B CFO. Someone with them who is experienced in building models like this will have a good template to start from should be able to put together one for your business in 5 - 8 hours on an hourly basis. If they are quoting more than that, find another fractional resource.
My firm, Venturity, can also do this for you. We're an outsourced accounting firm with higher level resources that can tackle these sort of tasks quickly and effectively. We do this for a lot of our clients already, and because we're accountants, we truly understand projecting all three statements, not just the income statement. Happy to discuss at no charge, just reach out to me with a follow up.
It sounds like you're in the throngs of creating your business plan; or creating financial statements for potential investors. You should begin by looking for an accountant that understands the business environment you are operating in -- the tax environment, service vs tangible products, B2B vs B2C etc.
I'll begin by introducing myself: an accountant and entrepreneur of 20+ years, with varied experience, most of which has been with small businesses. I have created business plans for service and retail operations, and maintained books for micro- to medium-sized businesses.
Market research is the most important task for this -- find out what is "normal" for the industry your business is in, concerning things like typical sales volume, typical overhead costs, typical tax liability. Typically the internet will provide general data, and industry publications and websites can provide even more details. Previously-published prospectuses from past share offerings can give you a peep into current financials of a similar business. These bits of data will be used as a guide for your accountant, who will also have to apply professional experience.
More details about your business will be needed in order to guide you properly. I'll be happy to speak further with you; do call if you need more guidance.