How to make a scrollable menu like angry bird?

I'm trying to "clone" the scrollable and selectable menu system like angry bird. But, I don't know how to make it. My idea is: when the user click in to a level image, the selected level is centered and focus, and when the user click again, the app starts a new scene.

Now, I have the below code that represents three levels. I've used the 'window' and GUILayout. If I change GUILayout to GUI, the menu don't scroll, but when I use GUILayout the window shows nothing.

It my first menu creation and I don't know how to do that. How I can do this better and how I can do that?

 var window_size = 300;
 var window_padding = 100;
 
 private var index = 0;
 
 function OnGUI () {
 
 // First level
 GUILayout.Window (0, Rect ((Screen.width - window_size) / 2 + (window_padding + window_size) * (0 - index), (Screen.height - window_size) / 3, window_size, window_size), DoMyWindow, "Universe 1");
 
 // Second level
 GUILayout.Window (1, Rect (((Screen.width - window_size) / 2) + (window_padding + window_size) * (1 - index), (Screen.height - window_size) / 3, window_size, window_size), DoMyWindow, "Universe 2");
 
 // Third level
 GUILayout.Window (2, Rect (((Screen.width - window_size) / 2) + (window_padding + window_size) * (2 - index), (Screen.height - window_size) / 3, window_size, window_size), DoMyWindow, "Universe 3");
 }

 // Make the contents of the window 
 function DoMyWindow (windowID : int) { 
     switch(windowID) { 
     case 0: 
         Debug.Log("1");
         index = 0;
         break; 
     case 1:
         Debug.Log("2");
         index = 1;
         break;
     case 2:
         Debug.Log("3");
         index = 2;
         break; 
     } 
 }