First you add a file entry for your sheet into your configuration
file.
Lets say you want to call the new SPIP sheet file
mysheet.spip, then you have to add the entry <file>mysheet</file>
in the <files> section of your
config.spip.
Then you create a new file mysheet.spip in the sheet
directory (or you copy it from an existing sheet file for
saving you from typing unneccessary many characters).