This is kind of a complicated answer so I will just paste "How to Use" from the customer section.
TLDR you need the cheat.
A new feature added to B4B Cheat that will allow you to draw multiple cards for each round/act/level.
Make sure the feature is ON in Misc. Then go join a match, After the Corruption cards screen you should have 5 card draws that will not end.
This will continue until you turn the feature off. So each level you will be able to select multiple cards.
WARNING:
You can select up to 255 cards in total when it comes to Online play. Above 255 per map, the server will disconnect you and you won't be able to join back to that game.
Stacking more than 400+ cards will start affecting the shitty servers of TRS. The game will become extremely laggy. (You have been warned.)
Once you feel like you have enough cards for that ACT turn the feature off. It is not recommended to have more than 400.
After the ACT 6 update.
Inf Deck Size:
With the recent update of ACT 6 added to the game, devs got a wee bit smart and tried to patch the Deck Size modification as much as they could.
Now, for you to play online with your 15+ Custom Deck, you first need to create a deck and play 1 session offline with it. Get a checkpoint by getting to a safe room which will save all your progress including your cards. After that, you can go ahead and play Online with the cheated Deck.
Inf Card Draws:
This works as before, you can choose up to 255 cards infinitely (Generic ones) and cannot exceed 255, however, if you are doing it online make sure you are not picking too many. If you get kicked this means you picked too much and won't be able to join back to that map, server will kick you on reconnection.