Install the library from the processing ide, here’s a simple sketch that demonstrates how to store current settings as json, NB: use of the data_path wrapper will save much misery.

load_library :controlP5
include_package 'controlP5'
attr_reader :cp5

def settings
  size(400, 400)
end

def setup
  sketch_title('Test Control')
  @cp5 = ControlP5.new(self)
  cp5.add_slider('s1')
     .set_position(20, 100)
     .set_size(200, 20)
  cp5.add_slider('s2')
     .set_position(20, 130)
     .set_size(200, 20)
     .move_to('extra')
  cp5.load_properties(data_path('default.json'))
end

def draw
  background(20)
end

def key_pressed
  return unless key == 's'
  cp5.save_properties(data_path('default.json'))
end

Heres an example json file:-

{
  "/s1": {
    "min": 0,
    "max": 100,
    "value": 48.5
  },
  "/s2": {
    "min": 0,
    "max": 100,
    "value": 75
  }
}