Genesis comes with the sample child theme which has an empty css file and an empty functions.php file.
Start there. Edit those files to do whatever you like.
See the tutorials here:
It would be helpful to know what exactly you are trying to do as well. ;) Do you want change the footer? Then look at the footer tutes. Do you want o do things to the header? Then you look at the ones we have for modifying the header.
Creating your own child theme follows the same process as editing an existing theme to do what you like.
These forums were CLOSED to new posts on November 19th, 2012. New forums are here: http://www.studiopress.com/forums/
and you will need to SIGN UP AGAIN.
If you need assitance with setting up your theme, go to http://my.studiopress.com/
and read the SETUP links or the TUTORIALS link on the side tab.
If you need Account Assistance
, file a help desk ticket at the same location.