I started with a basic understanding about the parts involved, but not so much how they fit together. Most of the necessary information came from the lwn series of articles posted in another comment: https://lwn.net/Articles/531114/

The actual namespace stuff was easy, the harder part was pivoting the root fs and figuring out all the things to mount. At some point I looked at the source for systemd-nspawn, but I forget exactly what for.