I spent some time this week learning Extensible Stylesheet Language (XSLT), which is used for styling XML documents. Another way to think of it: XSLT is to XML as CSS is to HTML. Moreover, if you have HTML/CSS knowledge, picking up XSLT is not too hard.
I got started with this video, which shows a very clear example of how to use XSLT:
Another resource that will be helpful to beginners is the w3schools XSLT site.