I read something about how to do it in this post by The merbist, albeit the information was not complete.
So here are my instructions:
Clone mislav’s will_paginate repo and build the gem from the agnostic branch.
With this commands you’ve generated the will_paginate-3.0.0.gem.
If you want to install it system-wide use your usual gem install.
If you want to bundle it in your project, as I did, then you’ll have to do: