Your question is ARIMA Forecasting Methodology. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
How would you approach building a predictive model, such as an ARIMA model, to forecast a macroeconomic indicator like GDP? What steps would you take to ensure out-of-sample accuracy?