Present Arms swaps your library with another deck you own from outside the game and use that one instead ("library" and "deck" both refer to the same thing, but the former is used to refer to it in-game, whereas the latter is used to refer to it outside of the game)
Make sure your main deck contains exactly the number of cards required for your format (typically 100 for Commander, while Standard, Modern, and Pioneer have 60)
Do keep in mind, however, that most of these combos require a lot of mana and cards to set up and they're pretty easy to shut down
(7/13/2016) You can't target one creature twice to give it -26/-26.
I swing in, going to 14 against his 13, cast Portent on my opponent, seeing Stifle, Island, and Impulse
cards, this Character gains +1000 power