you do this by creating a butane config yaml and converting it to an ignite json using butane.

dnf install -y butane

Create a yaml file with your config, mid the indentation. In this example only a group and a user with a specific uid is created (an exhaustive list of the options for butane config can be found here) i name this file version1.bu

version: 1.6.0
passwd:
  groups:
    - name: beheerders
      gid: 700
  users:
    - name: abel
      uid: 700
      primary_group: users
      groups:
        - beheerders
      ssh_authorized_keys:
        - ssh-ed25519 AAAAC3................P abel@nergens

Then with butane transform it ti the ignite json: butane –pretty –strict version1.bu > version1.ign

the resulting json file version1.ign looks like:

{
  "ignition": {
    "version": "3.5.0"
  },
  "passwd": {
    "groups": [
      {
        "gid": 700,
        "name": "beheerders"
      }
    ],
    "users": [
      {
        "groups": [
          "beheerders"
        ],
        "name": "abel",
        "primaryGroup": "users",
        "sshAuthorizedKeys": [
          "ssh-ed25519 AAAAC3N................P abel@nergens"
        ],
        "uid": 700
      }
    ]
  }
}

Copy the previously made ignite file to a separate USB disk for use in installation later.

Download the fedora coreos image from :

https://fedoraproject.org/coreos/download?stream=stable&arch=x86_64

and create a bootable installation medium USB stick using dd or Rufus

stick the USB install disk in the server, turn on and in the BIOS or in the bootmenu choose the USB device as boot device.

once the system is booted on the promt enter:

lsblk

and choose the disk to install to (in this example /dev/sda). Then start the installation :

sudo coreos-installer install /dev/sda --ignition-file version1.ign

https://www.flatcar.org/docs/latest/provisioning/config-transpiler/configuration/

https://docs.fedoraproject.org/en-US/fedora-coreos/bare-metal/

https://www.redhat.com/en/blog/red-hat-enterprise-linux-coreos-customization