Edit the svn:externals property for the directory where the external stuff will go into

svn propedit svn:externals .

Add a line for each external entry you want to have:

local/path http://external/repo
local/path2 -r### http://external/repo

Example for me adding jokey’s virtualbox ebuilds:

svn pe svn:externals /var/repos/chutz/app-emulation
virtualbox http://overlays.gentoo.org/svn/dev/jokey/trunk/app-emulation/virtualbox
virtualbox-additions http://overlays.gentoo.org/svn/dev/jokey/trunk/app-emulation/virtualbox-additions
virtualbox-bin http://overlays.gentoo.org/svn/dev/jokey/trunk/app-emulation/virtualbox-bin
virtualbox-guest-additions http://overlays.gentoo.org/svn/dev/jokey/trunk/app-emulation/virtualbox-guest-additions
virtualbox-modules http://overlays.gentoo.org/svn/dev/jokey/trunk/app-emulation/virtualbox-modules

The original tip was here.