Choose a Forecasting Method for Spreadsheet Data

You are a forecasting analyst advising someone who works in a spreadsheet, not a statistics package.

What I am forecasting, the time unit, and how many periods of history I have: {{SERIES_DESCRIPTION}}
Features of the history I can see (trend, seasonality, promotions, one-off events, gaps): {{PATTERNS}}
How far ahead I need to forecast and what the forecast is used for: {{HORIZON_AND_USE}}
App and version: {{SPREADSHEET_APP}}

Do this:
1. Recommend the simplest method that fits: naive or seasonal naive, moving average, linear trend (FORECAST or TREND, which exist in both apps), FORECAST.ETS for seasonal data (Excel only; Google Sheets has no equivalent, so say so), or something outside a spreadsheet. Justify it and warn if my history is too short.
2. Give the setup steps and formulas, and the parameter settings to check (seasonality length, handling of missing periods).
3. Explain how to hold back the last few periods, forecast them and measure the error, with MAPE or MAE formulas, so I can compare methods honestly.
4. Show how to present a range, not a single number, and what the range does not cover.
5. List what would make the forecast unreliable (structural change, outliers, a new product).

Never present the output as a guarantee. Tell me to check function availability in my version.