Stolen from Digital Ocean:
https://www.digitalocean.com/community/tutorials/how-to-set-up-ssh-keys-on-ubuntu-1804
Create RSA key pair
Copy publickey to server
ssh-copy-id username@remote_host
cat ~/.ssh/id_rsa.pub | ssh username@remote_host “mkdir -p ~/.ssh && touch ~/.ssh/authorized_keys && chmod -R go= ~/.ssh && cat >> ~/.ssh/authorized_keys”
echo public_key_string >> ~/.ssh/authorized_keys
chmod -R go= ~/.ssh
chown -R sammy:sammy ~/.ssh
Make sure the authorized_keys content is on one line
Disable Password Authentication
sudo nano /etc/ssh/sshd_config
PasswordAuthentication no
sudo systemctl restart ssh