NB: I realize your 1020 is not included in the list of HP printers in CUPS, but maybe the hp-laserjet_1022-hpijs-pcl.ppd.gz driver will work. Give it a try, and if not, just remove the printer with "Remove Device" in the hp-toolbox menu.
Even better than the CUPS web interface, IMO, is "hp-toolbox". From Slackware-12.1/CHANGES_AND_HINTS.TXT:
There is a minor problem with the HPLIP and CUPS versions in Slackware 12.1;
hp-toolbox will not work unless LC_ALL is set to a UTF8 locale.
An easy workaround is to start it with "LC_ALL=$LANG.UTF8 hp-toolbox" if
you're not using a UTF8 locale. Also, your user account must be a member
of the "lp" group for hp-toolbox to work properly, and to use the scanner
portion of some (all?) HP print/scan/copy units, you'll need to be a member
of the "lp" group. This is due to the fact that hplip's udev rules set
the device with group "lp" ownership.
My results with hp-toolbox on several computers running Slack-12.1 has been better than CUPS web interface on all previous versions.
First, as a normal user (not root) on your box, check if you're in the lp group by issuing "id":
uid=1000(mingdao) gid=100(users) groups=7(lp),10(wheel),11(floppy),17(audio),18(video),19(cdrom),83(plugdev),93(scanner),100(users)
If not, then su to root and issue "vigr" and add your normal user to the lp group.
Second, check to see if you are using a UTF-8 locale (you should know this) by issuing "echo $LANG":
mingdao@silas:~$ echo $LANG
If not, the easiest thing to do is to create the file .profile in your home directory with these settings:
mingdao@silas:~$ cat .profile
Or, if you don't want a UTF-8 locale, as the guide says, start hp-toolbox with "LC_ALL=$LANG.UTF8 hp-toolbox".
Third, when you start hp-toolbox, if your printer is not found, you will need to scroll down in that dialog box and start it per the instructions.
There are quite a few command line tools for HP printers:
hp-align hp-clean hp-fab hp-info hp-makecopies hp-print hp-scan hp-setup hp-testpage hp-toolbox
hp-check hp-colorcal hp-firmware hp-levels hp-makeuri hp-probe hp-sendfax hp-systray hp-timedate hp-unload
From the hp-toolbox dialog, you can also print, scan, fax, and adjust all the settings for your printer. IMO it is much better than the CUPS web interface.