nodm is an automatic display manager which automatically starts an X session at system boot. It is meant for devices like smartphones, but can be used on a regular computer as well, if the security implications are acceptable.
Now ensure no other display managers get started by disabling their runit services.
After installing nodm, you should modify the /etc/sv/nodm/conf file.
Now set the NODM_USER variable to the user that should be automatically logged in, and change the NODM_XSESSION variable to point to the script that starts your session. This script must be executable!
sudo ln -s /etc/sv/nodm /var/service