Customizing Purchase Order XSL template file

Hello,

I'm currently trying to find information to modify the standard purchase order stylesheet PO_STANDARD_XSLFO.xsl.

I mean, I know how to download it, I know how to update it when modify on OEBS, but what I'd like to know is how to modify the XSL file, to understand how it works exactly, how to use the BoilerPlate and how to call variables and add new column, new information....

If someone have some documentation on this please, will be very usefull...

Thanks in advance,

Stéphane