You have to map cube 3D coords into screen 2D ones, you may choose, for instance, perspective transformation or (the simpler) isometric one. :)
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke