In order to access the War Avatar window open your bag and click on the War Avatar button.
You will see initially six empty slots. You can equip cards on those slots, one card per slot. Also, each slot offers different bonuses.
Destroyer slot: Physical + Magical Attack
Battle slot: Physical + Magical Attack
Longevity slot: Health + Physical Defense
Durability slot: Health + Magical Defense
Soulprime slot: Spirit
Lifeprime slot: Spirit
If you put your mouse over a card you will see the card says something like "Type Battle" therefore you should equip it on the Battle slot. You can equip the cards in any slot you wish but you won't receive the card's full bonus if you do not equip them in the correct "Type" slot. You will receive the base stats of said slot but you won't get any additional bonuses (blue adds on the cards or leveled up bonuses) or set bonus. Each card type should be equipped to the corresponding slot for best results if possible. If you equip a card, you can un-equip it. They work like gear pieces.
There are currently 218 available cards to play around with. In order to check them out, access your War Avatar window and click the bottom right button "War Avatar Book". It seems that you can only check the stats of cards you have acquired previously using the book.
In order to find your cards, click the bottom right middle button "War Avatar Bag". Cards will automatically be placed in the bag, when packs are opened. You can drag & drop cards from the War Avatar Bag to your inventory and vice versa, or to card slots.
Cards come in grades. The currently known grades are C, B, A, S and S+ from weakest to strongest.
Apart from that, cards have a secondary level which you can level up yourself. Leveling a card up will increase its base stats (attack or defence or spirit) but it does not affect additional bonuses (displayed in blue letters).
In order to level up a card you will have to make the card devour another card. The devour button is at the bottom center of your War Avatar window. Select the card you want level up by simply clicking on the card slot or your equipped card. Have your War Avatar Bag open and click the Devour button. Once you do it, your cursor will change shape. Simply select the cards that you wish to get devoured by clicking on them. Devoured cards will disappear. You cannot feed higher level cards to lower level cards.
You can also choose to make a card devour a War Avatar Box (the box that contains a random card). You cannot devour S cards or S+ cards, so must devour S packs to obtain the points, or open the packs and lose the points if trying to obtain the associated "Titles".
Additional Things to Know About the Cards.
Some cards have a "set bonus". Just like gear, collect all cards and equip them to the correct "type" slot to get the set bonus. Other than S+ and S cards from the boutique, cards from packs are random, so you need luck to get all the cards of one set. It's generally better to opt for sets. Card sets increase the (white) stats of the cards.
When you are checking a card out, it will display your own class at "Current class" regardless of which class is using the card. Cards are bound to the character. They cannot be traded or account stashed. You can't drop or trade cards either. Some card stat multipliers are affected by the class using it.
Cards can be reawakened. This can be done at the War Avatar Master NPC in Kirin Town, Primal World.
When a card has reached max level you can consume another (of the same type) card to reawaken your max leveled card. Just like characters, a reawakened card will go back to level 1 but its base stats and growth will be higher. At lvl 66 a reawakened card has equal stats to the lvl 100 unawakened version of it. lIf you want to reawaken a card for the second time, you'll have to feed it a reawakened (once) card.
A card can be reawakened two times at most. The first reawakening will offer 130% of base stats and the second will offer 160% of base stats.
The basic rule is:
To RB1 a card needs a lvl 100 card and a lvl 1 card.
To RB2 a card needs a lvl 100 RB1 card and a lvl 1 RB1 card
Therefore an RB2 card requires 4 cards in total
(A lvl 100 card and a lvl 1) makes the RB1 card which is then maxxed out to lvl 100.
(A lvl 100 card and a lvl 1) makes the RB1 card which is then used to reawaken the prior RB1 lvl 100 card.
***DO NOT LEVEL THE 2ND RB1 CARD AS IT IS WASTED TIME AND POINTS***
Each character starts with 10 base Leadership. In general, the cards have a set amount of leadership required in order to be equipped. For example if you equip six C cards that all require "Leadership 1" you will have to use 6 points out of 10 from your Leadership to equip them.
Higher grade cards will typically require higher leadership. The amount of leadership is not grade exclusive though and it may vary among cards of the same grade and same type. The minimum leadership a card can have is 1 and the maximum leadership a card can have is 3.
Overall, the better cards you get, the more leadership you'll need so you'll have to eventually increase your leadership. To do this, you'll have to buy War Avatar Cataclysts from the boutique, or farm them in Mega FCC, and talk to the War Avatar Master NPC in Kirin Town, Primal World.
The amount of Catalysts required to increase leadership depends on the current amount of your leadership. You need 10 to increase it to 11, you need 20 to increase it to 12, you need 30 to increase it to 13 and so on.