Use of libFFI with Ruby is done via the Fiddle library …
You kind of need to poke around the Fiddle docs and modules to see examples.
It can also help searching the web for examples posted in blogs, etc.
How much experience do you have with Ruby ? … and writing C extension for Ruby ?
If you have not much I recommend downloading the PDF book on extending Ruby with C.
You can find it in the Books list, at the bottom of the “Downloadable” section …
Also see the SketchUp Extensibility Team’s examples …