# apt-get install arcanist
$ arc set-config default https://forge.softwareheritage.org/ $ arc install-certificate
arc will prompt you to login into Phabricator via web (which will ask your personal Phabricator credentials). You will then have to copy past the API token from the web page to arc, and hit Enter to complete the certificate installation.
All done! Now have a look at arc help and start hacking.
Arcanist configuration is stored in ~/.arcrc.