works for lexmark z603 on 8.04.1 (should work for other printers as well)
this is how i did it, starting on home folder (defalut if you open from Applications>Accessories>Terminal
i then rebooted my pc. when it booted up i open the terminal again and
$ sudo apt-get install alien libstdc++5 # libstdc++5 was suggested as needed for compatibility issues
$ mkdir lexmark # creates subfolder named lexmark
$ mv CJLZ600LE-CUPS-1.0-1.TAR.gz lexmark # moves the package folder lexmark
$ cd lexmark # goes to said lexmark
$ tar -xvzf CJLZ600LE-CUPS-1.0-1.TAR.gz # extracts the driver.
$ tail -n +143 z600cups-1.0-1.gz.sh > install.tar.gz # extracts the binary portion of the script.
$ tar -xvzf install.tar.gz # extracts the contents produced by tail
$ sudo alien -t z600cups-1.0-1.i386.rpm # converts unusable rpm packages to tgz. ignore script-related warnings.
$ sudo alien -t z600llpddk-2.0-1.i386.rpm # converts unusable rpm packages to tgz. ignore script-related warnings.
$ sudo tar xvzf z600llpddk-2.0.tgz -C / # extracts the tgz's to / putting the files in their right place
$ sudo tar xvzf z600cups-1.0.tgz -C / # extracts the tgz's to / putting the files in their right place
$ sudo ldconfig # DO NOT SKIP THIS STEP or your printer backend won't find required libraries
$ cd /usr/share/cups/model # changes folder where you are operating
$ sudo gunzip Lexmark-Z600-lxz600cj-cups.ppd.gz # unzips the ppd, which should _not_ be gzipped
i returned nothing, but i guess it means something. i then rebooted again, just to make sure. then opened System>Administration>Printing.
$ cd /usr/lib/cups/backend
$ sudo ./z600
selected Z600 Series (it _was_ there) but printing tests didn't work. so i chose offer ppd file, and looked for the one on /usr/share/cups/models and when it asked whether i wanted to keep configurations, i chose not to. tried printing test page and it worked.