if you have been running your business for sometime then you should use your historical numbers to forecast both sales and expenses, and you should have a reasonable idea about the seasonality of your business

make sure you create a spreadsheet to forecast your monthly cash flow, where I am assuming most of your sales are either cash or by credit card so the proceeds should be in your account within 24 hours or so. On the other hand you should have an idea of what your direct cost of sales is ( for example what you sell for $100 cost you $60, meaning your cost of sales is 60%). Accordingly factor this percentage on your spread sheet, which will give you an idea of what your monthly cost of sales is.

Also add all non direct expenses, things like rent, payroll, hydro, insurance, internet, interest...etc

now your monthly numbers should show you what is the net cash you have for each month, and accordingly you can make decisions

This information can help you arrange a line of credit with your bank to cover those months where you are short in cash. Also try to negotiate better payment terms with your suppliers, 15/30/45 days or so, which gives you room to manage your cash.