dragonfly_initializer.rb

# ... other initialization stuff above here ...
 
Dragonfly[:images].processor.register(Watermark)
 
# optional, but gives you the ability to use watermark(opacity: 50)
# rather than process(:watermark, opacity: 50) 
Dragonfly[:images].configure do |c|
  c.job(:watermark) { |*args| process(:watermark, *args) }
end

@luopio Yes, the API has changed since the writing of this gist. Thanks for sharing your version.