Using the Preview Add-on for Ckeditor

I have the 'ckeditor' gem installed, but I'm trying to also add a
preview pane for users to see the output of their HTML
document/formatting. The gem's documentation tells you everything but
how to add new plugins.

Can you share with me your approach if you've used the gem before?

Adding config.extraPlugins = 'preview' to 'ckeditor.rb' initializers
doesn't work.

I went into the gem's directory to add a plugins folder
app/assets/javascript/ckeditor/plugins.... I then added preview.js to
it. Afterwards, I added //= require ckeditor/plugins/preview.js to
ckeditor\application.js. Lastly, I added config.extraPlugins =
'preview'; to config.js. I still don't see the preview button or preview
pane for the gem.