Start with the basics - how much do your materials cost for a days work and the cost of any processes that use power so for example what would the electric cost for a day? Do you pay anything to advertise? All of that together will give you your daily expenditure. Or you can work it out per bar of soap if that’s easier.
Then work out how much soap you can make in a day, how much you’ll sell, and what you charge for it, and that’s your income.
How do they sit one against the other? Are you able to sell enough soap to cover your expenditure, or do you need to make/sell more soap in a day to cover it? Or does the soap need to be more expensive, and does the quality justify that?
As a start-up you often work for nothing for a while to get the business going, but within a few months, say at six months, you need to factor in what you’d like to take as wages; start at minimum wage (it’s on gov.uk) and factor that into the expenditure as well.
That’s a basic business plan - what does my product cost me to make, what can I sell it for and how much of it do I need to sell to pay myself a reasonable wage?
Do that for a while and once it’s all humming along nicely, think about how you might expand- more soap, different soap, where to sell it and what that would cost. Or you might be happy with just humming along and that’s fine too, a business plan can be about staying the same once the figures work.
Best of luck, I work for myself and I love it!