On the host from which you will administer the CoreOS nodes from (aka the "admin machine"), make sure to copy (or generate new) SSH public key which will be used for authentication to the CoreOS machine(s), so in case there is no public key exist:
ssh-keygen -t rsa -N '' -f ~/.ssh/id_rsa
Boot your machine with any Linux LiveCD (with internet connection ;) ).
Edit a cloud init file - which is basically a YAML that describes how the CoreOS machine is going to be installed & configured, at the very minimum it should contain the public key we have previously generated/copied:
#cloud-config ssh_authorized_keys: - ssh-rsa AAAblablabla
Save and fetch CoreOS install script:
wget -O core-os-install.sh https://raw.githubusercontent.com/coreos/init/master/bin/coreos-install
Run the install script (this will wipe out /dev/sda of course):
bash ./core-os-install.sh -d /dev/sda -C stable -c cloud-init.yaml
When the installation is done, boot to the new CoreOS kernel and try to login as user 'core' with the public key provided in the YAML above.