* HPLIP communication with the printer is done via CUPS so you need to
give the _cups user r/w access to /dev/{usb,ugen}*

* the printer must attach as a ugen(4) device which means you need to
disable ulpt in your kernel - see config(8).

* to provides persistent data and event services to HPLIP client
applications, you must start the hpssd daemon (it needs to be done
before the cupsd spooler is started).  As root:
 ${PREFIX}/sbin/hpssd

* to make the hpaio backend available to SANE, simply add "hpaio" to
its configuration file.  As root:
 echo "hpaio" >> ${SYSCONFDIR}/sane.d/dll.conf