Start from operating-income, tax it at the effective-tax-rate, add back non-cash-charges, subtract capex and the change-in-working-capital. Because interest never appears, the result is independent of how the company is financed.
This is the stream discounted at wacc in a standard discounted-cash-flow model. The result is enterprise value, from which net-debt is subtracted to reach equity value. Mixing levered cash flow with WACC is the most common modelling error.
Example: Northwind Tools has $120M of EBIT, taxed at 24% gives $91M, plus $75M of D&A, less $75M of capex and $21M of working capital build, leaves $70M of unlevered free cash flow.
Related: discounted-cash-flow, wacc, free-cash-flow, operating-income, net-debt