Each input comes from the cash flow statement. Operating cash flow, tagged NetCashProvidedByUsedInOperatingActivities in XBRL, already reflects interest paid and taxes under US GAAP, so debt holders' interest has been taken out. Capital expenditures, usually tagged PaymentsToAcquirePropertyPlantAndEquipment, are subtracted. Net borrowing is proceeds from debt, such as ProceedsFromIssuanceOfDebt, minus repayments, such as RepaymentsOfDebt. The result is the cash left for shareholders after the company has dealt with its lenders in the period.
The net borrowing term makes FCFE volatile. A company that issues a large bond in one year shows a big jump in FCFE even though its business did not change; the following year's repayment does the opposite. For valuation, analysts often normalize borrowing to a target debt ratio rather than use one year's actual figure. Some definitions also subtract preferred dividends, since those come ahead of common shareholders, and some deduct acquisitions or finance lease principal. Because it is non-GAAP, a company that reports FCFE in its filings must reconcile it to the nearest GAAP measure.
FCFE is the cash flow used in equity-based discounted cash flow models, where it is discounted at the cost of equity to estimate the value of the shares directly. It is also a useful check on payout policy: dividends and buybacks that consistently exceed FCFE must be funded from cash reserves or new equity, which cannot continue indefinitely.