I know it really does not seem like it works but I'll explain in bit by bit.
Firstly you will need two
Sphinx's Tutelage out on the battlefield. The soonest you will be able to do this will be turn four. So what that means is that when you draw a card your opponent will have to put at least the top four cards of their library.
Once this is set up you then go ahead and cast the
Day's Undoing this will rest their grave, yes this seems bad but they will also be drawing 7 cards and you will be doing the same. This will also trigger both of your
Sphinx's Tutelage so they will have to put the top four cards of their library 7 times (one for each card you draw from
Day's Undoing) that is a total of 28 card and that is excluding the 7 they had to draw from
Day's Undoing and saying if two cards that are being milled don't share a colour and or its a land. The later in the game you use this combo the better because they would have played lands so it is less likely that lands will be milled.
Day's Undoing also helps if you need to fuel
Zada, Hedron Grinder if you have none.
Remember it is resetting your grave as well so any draw cards that you have already used you have the possibility of using again.
I hope this helped you understand. If you still don't see it I say test the deck out and you will see for your self