1. Drag Tk2DROOT/tk2dUI_demo/ControlPrefabs/Layout/ScrollableArea_Horizontal / Vertical into the scene. Make sure the newly dragged prefab is selected, and in the scene view, resize to the size you need this at.
2. Drag the contents of the scroll view into "Content" - Don't change the position of the object, but position your items underneath it.
3. Click "Calculate content length" in the scrollable area inspector to make it fit the content, you should see the blue content length widget resize to fit the contents you added.
That should be it.