CoreOS is badly documented, but you can start here to read about installation:
   https://coreos.com/os/docs/latest/installing-to-disk.html#container-linux-configs

It is very wrong about one key command. It's:
   coreos-install -d /dev/sda -i ignition.json
And not:
   coreos-install -d /dev/sda -c ignition.json

Beyond that, the page seems reasonable.


Other notes:
 - Password hashes seem to be standard linux hashes.


Things I did to these machines:
 - sudo systemctl start fleet
 - sudo systemctl enable fleet


Adding kubernetes:
 - MASTER_HOST=18.181.0.106
 - ETCD_ENDPOINTS=http://18.181.0.97:2379,http://18.181.0.104:2379,http://18.181.0.106:2379
 - POD_NETWORK=10.18.0.0/16
 - SERVICE_IP_RANGE=10.19.0.0/24
 - K8S_SERVICE_IP=10.19.0.1
 - DNS_SERVICE_IP=10.19.0.2