# apt-get install arcanist
$ arc set-config default https://USERNAME:PASSWORD@forge.softwareheritage.org/
where USERNAME/PASSWORD are the general credentials used to access the various Software Heritage websites. (No username/password will be needed for this step in the future, once Software Heritage goes public.)
$ 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.