Here is an example:
iGrid1.Header.Cells[0, 0].TextFormatFlags = TenTec.Windows.iGridLib.iGStringFormatFlags.DirectionVertical;
You may also want to use the iGStringFormatFlags.DirectionVertical flag together with iGStringFormatFlags.Rotate180 if you do not like the default orientation.
After setting these flags for the required column headers, call the iGrid1.Header.AutoHeight() method to automatically adjust the header height to display all column header texts without clipping.