A few months back, I altered my twentyten theme to change the header image (to something of the same width but a bigger height than strictly allowed in the theme template) and to add a logo in place of the words on top. I found a fantastic thread which gave detailed instructions but despite my best efforts I can't seem to find it again.
I didn't understand the concept of child themes then so the changes have since been overwritten. I have already set up a child theme style.css but I have no idea how to alter it. I remember having to alter both the style.css as well as the header.php.
If someone could walk me through the process, I'd be very grateful.