For future reference. Taken from here, shortened version.
##### Create a user
adduser sammy
usermod -aG sudo sammy
##### Copy ssh keys
su - sammy
mkdir ~/.ssh
chmod 700 ~/.ssh
vim ~/.ssh/authorized_keys
# Insert public key from the local machine, cat ~/.ssh/id_rsa.pub
chmod 600 ~/.ssh/authorized_keys
exit
##### Disable password authentication
sudo vim /etc/ssh/sshd_config
# Set: PasswordAuthentication no, PubkeyAuthentication yes, ChallengeResponseAuthentication no
sudo systemctl reload sshd
##### Upgrade and install packages
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install python3-pip python3-dev libpq-dev postgresql postgresql-contrib nginx
##### Firewall
sudo ufw allow OpenSSH
sudo ufw enable
# Good time to check ssh connection works