If you've attached a USB printer to your machine and started the USB stack and devu-prn as described in "USB devices" in the Connecting Hardware chapter, you should set up the /etc/printcap file to be something like this:
hpps: \ :lp=/dev/usbpar0 :sd=/usr/spool/output/hpps
This file gives the name hpps to the USB printer, identifies the file to open as /dev/usbpar0 (or whatever device devu-prn created), and identifies the spooling directory as /usr/spool/output/hpps.
To access this printer, specify lpr -Phpps or set the PRINTER environment variable to hpps.