git clone https://github.com/mtigas/1pass2keepassx.git
cd 1pass2keepassx
bundle install

Using: Current versions of 1Password

(As of August 10, 2014.)

Note that newer versions of 1Password actually use a folder as the .1pif "file" and the actual legacy PIF-format file is inside it. So you'd want to do something like this:

bundle exec ruby 1pass2keepass.rb ${YOURFILE}/data.1pif

That'll spit out the KeePassX XML to your console. What you probably want is to save this to an XML. So redirect the output.

Putting it all together, you'd do something like this:

bundle exec ruby 1pass2keepass.rb ~/Desktop/pass.1pif/data.1pif > ~/Desktop/keepass.xml

Using: Older versions of 1Password

Basically like above, but directly accessing the .1pif file.

bundle exec ruby 1pass2keepass.rb $YOURFILE

#i.e.:
bundle exec ruby 1pass2keepass.rb ~/Desktop/pass.1pif > ~/Desktop/keepass.xml