First, set up Raspbian.
Install prereqs.
sudo apt-get install apparmor-utils apt-transport-https avahi-daemon ca-certificates curl dbus jq network-manager socat software-properties-common
Install docker.
curl -sSL https://get.docker.com | sh
Get some powers.
sudo su
Install Hass.io.
curl -sL https://raw.githubusercontent.com/home-assistant/hassio-build/master/install/hassio_install | bash -s -- -m raspberrypi3
The above was modified from the Hass.io setup guide.