TC was built manually, without scripts or environments. We do use busybox and tinyX, but normal glibc instead of uclibc. We don't have instructions for it, because that is not the goal of TC, and because there are several projects out there that do have a goal like that. Using them would probably make a lot more sense, as they do what they were designed to do very well (LFS, T2, DIY-Linux, others).
Our busybox config is at any mirror in the release/src and busybox tarball. TinyX is either built from a separate tarball (from before merging to X) or from the Xserver tarball (before 1.6, because they were removed between 1.5 and 1.6; if you wish to build one from there, I recommend one from Xserver 1.2, because several crippling changes were made after that).
As to how Xvesa got so small, Jason here did a lot of testing with different versions and editions until we hit a sweet spot with both features and size.
There was an user who wanted to replicate TC from scratch, see this thread for info:
http://forum.tinycorelinux.net/index.php?topic=299.0