How to get started with svn:externals?

I'm looking for a succinct and well-written tutorial on using svn:externals.

I already know how to use them in a basic way myself, but I want a good article that I can link to when answering questions like this one that come up recently:

What to do with multiple projects depending on the same source?

I'd do it myself, but I don't use them often enough to want to stick my neck out and write a tutorial on it. Google was surprisingly unhelpful with this topic.