Both lxml and ElementTree have tutorials:

http://effbot.org/zone/element.htm

http://lxml.de/tutorial.html

Here is another tutorial that may server as a source for an intro:

http://infohost.nmt.edu/tcc/help/pubs/pylxml/web/index.html

And the general ET documentation is here:

http://effbot.org/zone/element-index.htm#documentation

In terms of licensing, I can't speak for any of the other sources, but as for the lxml documentation, feel free to copy any ET related parts of it that you see fit.

I think the lxml tutorial is gentle enough even for beginners to follow. Note, however, that it uses lxml specific APIs in some places. In the specific case of XPath, the examples should be easily replaced with ElementPath, though.