Base environment setup for IntelliJ made following the basic tutorial on scala-js.org
First, make sure you have the scala compiler installed. If you've never worked with scala, you probably don't have it. You can get it here
Clone the repo and import it to IntelliJ with sbt and make sure the use sbt shell for imports and for builds boxes are checked.
In the terminal, run npm install on the project root to install all of the node modules and dependencies.
Your html is at project/scalajs-tutorial-fastopt.html. You can use this as you would any other html or you can alter the DOM with scala at src/main/scala/tutorial.webapp/TutorialApp
You can rename any of these files to fit your project but be sure to change the paths and references in other files.
Go to your sbt shell on Intellij and run fastOptJS to compile the scala code into a js file. This will generate a file under the target/scala-2.12 folder. You call it in your html with a regular script tag. You can also turn on auto-compile if you run ~fastOptJS
If you are not using any other framework like React or Angular, you can simply open up the html file in your browser. If you are using a framework then you are on your own, good luck I hope you do well and let me know if you figure it out because I don't have a clue.
Tutorial I used to setup
Other tutorials by the official Scalajs site
Scalatags tutorial (very useful)