My complete steps to join a new Raspberry Pi 4 node running Ubuntu 19.10 to the cluster

Connect the micro-hdmi, keyboard, and power.

Wait for it to boot up.

Login. Default user/pass is ubuntu/ubuntu, but it will make you change it on first login.

sudo apt update
sudo apt upgrade

Change the hostname, edit /etc/hostname

Enable the boot cmdline groups, edit /boot/firmware/nobtcmd.txt and add cgroup_enable=cpuset cgroup_enable=memory cgroup_memory=1 to the end of the existing line (do not add newlines)

Reboot. Log in again.

Run ssh-keygen

From the master node:

copy over ssh key from master to the new node, just to make things easier:

ssh-copy-id nodehostname

copy over the node token from the master so that the new node can join the cluster:

scp /var/lib/rancher/k3s/server/node-token ubuntu@nodehostname:.

Now, back on the new node...

export K3S_TOKEN=$(cat node-token)
export K3S_URL=https://masterhostname:6443
curl -sfL https://get.k3s.io | sh -

Watch the logs, but the final message should be something like

systemd: Starting k3s-agent

Back on the master, run

sudo kubectl get nodes

And make sure the new one joined. That's it