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).
Your sheet file mysheet.spip have to look like this:
<?xml version="1.0" encoding="ISO-8859-1"?>
<spip>
<chapter>Title of the chapter</chapter>
<subchapter>Title of the subchapter</subchapter>
<picture>Optional picture</picture>
<data>
Data of this sheet (HTML code).
</data>
</spip>