back to Luke Du's ugly page.

• ssh-keygen -t rsa -b 2048 -v
• input ubuntu_user along with two enter
• mv ubuntu_user ubuntu_user.pem
• cp ubuntu_user.pub authorized_keys (if file exists, append to)
• Get ubuntu_user.pem in your client workstation and run ssh -i ubuntu_user.pem user@ubuntu

See original doc on web.