After importing the shipping file, open a new worksheet $($tab$)$ and call it Q$4$ Forecast.

To perform a forecast, first right$-$click on Invoice Date and convert to a Continuous value.

Add Invoice Total to the Rows line, and Invoice Date to the Columns line.

Expand the invoice date until sales are shown by month.

Add Invoice Date from Dimensions to the Filter pane, select Range of Dates, and set the date filter to include all invoices through September $30,2021.$

To add the forecast, click on the $$Analytics$$ tab next to the $$Data$$ tab towards the top$-$left corner of Tableau.

Under Model, drag and drop Forecast on the line chart.

Left$-$click on the flat forecast line, and press Edit to change Forecast options.

We only want to forecast $2021$ Q$4,$ so change to forecast exactly three months, ignoring $0$ months.

Change the Forecast Mode