Here are more specific instructions for creating a report for a specific customer, grouped by month over the last 3 years as you have requested in your support email.
1. Go to the Reports tab and select the Sales By Customer report.
2. Press the Default button in the toolbar to default any filters, grouping already applied to the report.
3. Press the Change Filters button in the toolbar to show the Filters dialog.
4. Make sure that the Paid filter is unchecked then select the Customer row in the Filters dialog and press the Choose button to select a specific customer for which you wish to see the report.
5. With the Filters dialog still open, select the Date row then in the All Date dropdown select Custom Range and choose the date range for the report (from 3 years ago to today).
6. Press Close the close the Filters dialog.
7. Press the Change Title, Columns, Grouping button to show the Customise dialog.
8. In the dropdown at the bottom of the dialog, change the dropdown from Customer to Month Raised.
You should now have a report of sales to the customer grouped by month over the last 3 years.