Official Docs

1. Install

console

yarn add stimulus 
mkdir app/javascript/controllers
touch app/javascript/controllers/index.js

#app/javascript/packs/application.js

import 'controllers'

#app/javascript/controllers/index.js

import { Application } from "stimulus"
import { definitionsFromContext } from "stimulus/webpack-helpers"

const application = Application.start()
const context = require.context("../controllers", true, /\.js$/)
application.load(definitionsFromContext(context))

2. See if it works

console

touch app/javascript/controllers/hello_controller.js

#app/javascript/controllers/hello_controller.js

import { Controller } from 'stimulus'; 
export default class extends Controller {
  connect() {
    console.log("hello from StimulusJS")
  }
}

in a view:

<div data-controller="hello">
  Anything
</div>

3. Log an action on a click

#app/javascript/controllers/hello_controller.js

  welcome() {
    console.log("click")
  }

in a view:

<div data-controller="hello">
  <div class="btn btn-primary" data-action="click->hello#welcome">log a click in console</div>
</div>

Now when you open the view, the div will call the hello_controller and add hello from StimulusJS in your browser console.