Here are some sections about it in the svnbook / TortoiseSVN manual:
This is the svnbook page on svn:externals. It is the most useful resource on the subject.
This one explains four ways to include sub-projects in the workflow (and in the process misses a fifth one: symlinks).
And this last one is a TortoiseSVN-specific page on how to import stuff in a repository (not directly related to svn:externals).