J. David Eisenberg has posted "Validating XML with RELAX", which he describes as "for the web designer who knows a little about XML".

RELAX, an XML schema language devised by Makoto Murata, has been growing in popularity since its presentation at XTech 2000 in February this year. It is seen as a simpler alternative to W3C XML Schemas, and is on the fast-track to becoming an ISO standard. RELAX is also now supported by Extensibility's XML Authority schema design tool.

Eisenberg introduces his tutorial as for the beginner to XML validation:

It's written for the web designer who knows a little about XML and has heard about validation, and is wondering what it's all about.

The concise tutorial covers validation both with a DTD and with RELAX.