Now as you may have notice, a game of cardfight! vanguard is very fast paced, the early game and mid games is in the first few rounds of the game, and then you reach the endgame(sometimes you may not even reach it), where everyone have a number of rearguards with a grade 3 vanguard. The end game would proceed from there on. So now, about Imagine!, which was used by Kai in the anime, is to imagine how are you going to win the game. So how is this related to your deck? By knowing how you are going to win a game, you would know the cards that you would be using, and the cards that will help you create the possibility of that endgame scenario. Thus most of the cards that should be in your deck, the backbone of the deck, would be selected. The rest of your deck, you would use to support the endgame or try to bring about that specific scenario.
An example would be my granblue deck, which I may talk about in the future, has the imagine! Scenario that uses cocytus, necromancer of the icy prison boosted by dragon spirit to reach 26k after limit break as a vanguard and aiming to crit the opponent for victory. So cocytus and dragon spirit will most likely be placed in my deck, along with 12 critical triggers. The rest of the cards will either help me slow the opponent down, increasing my defense, so that cocytus will be able to attack more times, thus activating that critical triggers, or help me get cocytus out as vanguard.
So as you can see, through using Imagine! You would most likely construct your deck from the top down, selecting the particular grade 3s you wish to use in the endgame, then selecting the supporting grade 2s, followed by the grade 1s. The triggers that you select, although may be affected by the type of deck that you play, would also depend on your play style. Of course you may also have a backup plan, which is always a good idea in case you are unable to create the main winning field, to fall upon, thus deciding the selection of more cards into your deck.
So what do you do after selecting the cards for your deck? Play test them! Play test to see if your endgame is easily achievable and able to bring you victory, if not tweak your deck to make it more efficient in creating the endgame field that you want and to make a stronger endgame field.
So now that you had learned about Imagine!, your deck your be stronger and you would be less likely to lose, next up I will discuss about another thing to consider during deck construction, the magic numbers!