Open a new workbook. Customize as you wish, custom formats, number of sheets, styles, any default evnt code or macros.
Save the file as an Excel template in the XLSTART directory with the name BOOK.XLT.
This will then be the default workbook for File>New, the Toolbar button New or CTRL-n.
Note that if you use File>New...Blank Workbookr you will get the Excel default workbook.