In particular, once you have the apt-transport-tor
package installed, the following entries should work in your sources list for a Debian system:
deb tor+http://vwakviie2ienjx6t.onion/debian stretch main deb tor+http://vwakviie2ienjx6t.onion/debian stretch-updates main deb tor+http://sgvtcaew4bxjd7ln.onion/debian-security stretch/updates main #deb tor+http://vwakviie2ienjx6t.onion/debian stretch-backports main