I followed LiveCDCustomization and managed to build a distro:
However it only boots with VirtualBox, and this guide is hopelessly out of date. Searching around the net, I found this handy tutorial on Legacy Bios, UEFI and SecureBoot ready Ubuntu Live image customization
Now I managed to get something sort of working. There are still problems and the guide is not complete.
However the question still remains- where can I find this info?
Where is the source repository for Ubuntu with the build guide?