This library allows you to build great UI with drag-and-drop simple.
It is abstracting you from mouse events and other low-level stuff.
You can operate high-level things such as draggable and droppable areas.

The idea of package API is you should be able to wrap elements with draggable dragMeta to add an ability to drag it.
The dragging object will get some meta information.
Also, you could wrap another element with droppable dropMeta,
so if you drop element over that element, the message YourOnDropMessage dragMeta dropMeta will be invoked.

At first, you need to initialize draggable state and function.
DnD.init helper returns initModel, subscription, draggable and droppable functions for your message wrapper and onDrop message.

type Msg
    = NoOp
    ..
    | OnDrop String Int
    | DnDMsg (DnD.Msg String Int)


type alias Model =
    { ...
    , draggable : DnD.Draggable String Int
    }


dnd = DnD.init DnDMsg OnDrop


model =
    { ...
    , draggable : dnd.model
    }

Subscriptions allow you to get drop event.

subscriptions : Model -> Sub Msg
subscriptions model =
    dnd.subscriptions model.draggable

View wrapper for draggable object, you could drag object wrapped by this helper

draggable
    : (Html.Attribute Msg)
    -> List (Html Msg)
    -> Html Msg
draggable =  dnd.draggable dragMeta

View helper for droppable area, you could drop object to this area, after that your on OnDrop message will be invoked.

droppable
  : (Html.Attribute Msg)
  -> List (Html Msg)
  -> Html Msg
droppable = dnd.droppable dropMeta

You can find simple examples here.
For more complex example check Chess Board.