The geoCable is a custom cable that interfaces the user port on the Commodore 64 to the parallel port on a printer.  I found this short article on how to make the cable HERE. Thank you Internet Archive for preserving this!  

Further information on this topic can be found HERE , HERE , HERE and HERE.  I've never built one of these cables, so I recommend cross referencing all the information until you're comfortable with your chosen solution.

Also, issue 72 of geoNews (August 1997) has a short article on page 3 about creating this cable.  

One last source for info on the geoCable - the CMB FAQ v3.1 - http://www.faqs.org/faqs/commodore/main-faq/part7/