Howto build a custom OS from scratch

I'm modernizing an embedded Linux distribution built from RHEL rpms and looking to replace our bespoke build system with coreos-assembler. I've read the READMEs and done the tutorial to build a FCOS image and run it. It just worked, nicely :) Now I've started the serious digging and I thought I'd ask for pointers before reading the entire source. (I'm starting with the coreos-assembler script and the fedora-coreos-base.yaml manifest.)

1. I'm not building a fedora derivative, rather a very locked down system comprised of systemd and a small set of applications, their libs, and the rpms that support them (and bash for debug only.) Is there a minimal manifest example?

2. I need to do some very custom stuff with image production and initial ram disk. I'm starting to read at cmd-build script. It may be a long slog from there ;-)

Thanks in advance for your help.
Chris
cc: @ashcrow

Start with the coreos-assembler script and fedora-coreos-base.yaml manifest, then follow the cmd-build script as the issue author planned. The request seeks pointers for a minimal manifest and custom image and initial-ramdisk production, but it does not define a specific implementation or completion criteria.