Hi Benjamin,
If you're not real xslt guru (me neither), try this
toolReal nice tool that can teach/mentor you in getting the right xpath expression.
And of course you need some data for your expression to operate on. Get the umbraco.config file from /data folder of your umbraco install, that file holds the complete xml representation of the content tree.
Good luck with it.
Oh btw, I see that you're from Belgium. Have you read
this? Are you interested in joining?
Regards,
/Dirk
level 1 certified - umbraco blog at
netaddicts.be - working on an integrated forum4umbraco