Firstly, you do not shuffle them into your deck. You place them on the bottom of your deck in whatever order you like.
Getting to your question: you have to place at least one card on the bottom of your deck in order to draw back up to 6. It is your choice if you place one or two cards. The only way you may place no cards on the bottom of your deck is if you have no cards in your hand at the time. So basicly, you have to put at least one card and no more than 2 cards on the bottom of your deck in order to use Cosmic Power.
Hope that it makes sense, it's late for me and I'm half delusional from being tired. Nah, I know what I'm doing, hopefully it'll help you.