Principally you can build the cash flow plan on the profit & loss (P&L) statement. However only cash relevant income and expenses have to be included i.e. no depreciation or changes in inventories. Cash receipts as pre-payments or loans received as well as disbursements like investments, redemption or dividends paid, that are not part of the P&L, need to be added. Different from a P&L income and expenses are planned when they are expected to be paid. Thus you have to assume payment terms for customer and vendor unless they do not directly result from contracts (loan, lease, employment contracts). This is a crucial task so you should base the plan on realistic assumptions and plan rather conservative than too optimistic. And do not forget to account for customer not paying on time.