Yeah. Well, I've only poked around in it a bit. If you know how to write functional JavaScript it's easy. The hard part is balance and coming up with a fun formula. From my limited experience with them, I know you need:

• The click function
  This is the first and only available action the player can take, initially (in most cases). Click to collect some kind of resource. Write a function like this:

  var points = 0; // Track total points here
  var bonus = 0; // If you use modifiers for higher points per click, use this
  var theClick = function(n) {
      if ( n > 1 ) {
          points = points + ( n * (1 + bonus));
      } else {
          points = points + (1 + bonus);
      }
  }  
  

  Then just apply that function to a button in your interface. The n argument is explained below in the repeated function.

• Resource(s)
  In my example above, points are your primary resource. You'll earn it from your theClick() function. You can get creative and come up with multiple resources (with multiple functions) to add more depth to your game. This is good if you want to add a rarer resource later in the game.

• The repeating function
  Next you need to write a function that repeats every X seconds. This is used if you have "clickers" in your game. Any kind of upgrade that does the clicking for you. In the above example, I've included an argument to your function. Use this argument with the number of clickers the player currently has. Example:

   var clickers = 3;
   setInterval(
       theClick(clickers), 1000;
   )  
  

  This will fire your function every 1000ms (1 second). It passes the number of clickers as the argument, which multiplies the effect of the function. In the above example, it will add 3 to your points every 1 second.

• Upgrades
  Upgrades add depth and interest to your game. This could be something like a percentage bonus to clicks, or doubling the effectiveness of your clickers.

Those are the core building blocks of any idle clicker game. The funnest games have many more layers to them, adding hundreds of upgrades, new resource types, animations, etc. It's a good exercise for someone who is an intermediate at JavaScript coding to build one of these kinds of games.

Edit: Fixed the click function.

What do you mean by "idle games"? Turn based? Casual games?