How exactly do you target a card in your deck to select?
You target a card on the field, and then draw a copy of that card from that deck.
Example -
I have Time Sage and Deja Vu on the field.
I use Memoir on the Deja Vu, and draw a Deja Vu from my deck.
Next turn, I attempt to use Memoir on Deja Vu again - except there are no copies of Deja Vu left in my deck so I don't draw another copy of Deja Vu.