Try this:
<?xml version="1.0"?>
<?mso-application progid="Excel.Sheet"?>
<Workbook xmlns="urn:schemas-microsoft-com:office:spreadsheet"
xmlns:o="urn:schemas-microsoft-com:office:office"
xmlns:x="urn:schemas-microsoft-com:office:excel"
xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet"
xmlns:html="http://www.w3.org/TR/REC-html40">
<ExcelWorkbook xmlns="urn:schemas-microsoft-com:office:excel">
<ActiveSheet>2</ActiveSheet>
</ExcelWorkbook>
<Styles>
<Style ss:ID="Default" ss:Name="Normal">
<Alignment ss:Vertical="Bottom"/>
<Borders/>
<Font ss:FontName="Calibri" x:Family="Swiss" ss:Size="11" ss:Color="#000000"/>
<Interior/>
<NumberFormat/>
<Protection/>
</Style>
</Styles>
<Worksheet ss:Name="Sheet1">
<Table ss:ExpandedColumnCount="2" ss:ExpandedRowCount="1" x:FullColumns="1"
x:FullRows="1" ss:DefaultRowHeight="15">
<Row>
<Cell><Data ss:Type="Number">11</Data></Cell>
<Cell><Data ss:Type="Number">22</Data></Cell>
</Row>
</Table>
</Worksheet>
<Worksheet ss:Name="Sheet2">
<Table ss:ExpandedColumnCount="2" ss:ExpandedRowCount="1" x:FullColumns="1"
x:FullRows="1" ss:DefaultRowHeight="15">
<Row>
<Cell><Data ss:Type="Number">11</Data></Cell>
<Cell><Data ss:Type="Number">22</Data></Cell>
</Row>
</Table>
</Worksheet>
</Workbook>
For more help you can take a look at this.
Finally, you can manually create an excel workbook and export it to XML so that you can examine the XML's format.
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