How do I break my game?

There are many ways to break the game. If you're talking just about breaking on the Chest, you can use Undefined to get to the i am error room over and over until you are pleased with your loadout. Most other ways involve something like Sharp Plug + Scapular, giving you infinite rerolls, or D20 shenanigans, or Jera/Perthro + Blank Card.

You don't need the d6.

The most common combo is the Rune of Jera (unlocked by beating high brow) and the Blank Card (unlocked by beating The Cathedral as Eden). This allows you to repeatedly use the rune with Blank Card.

Once you have these two, you need a room containing a battery or 48 hour energy pill. Use the blank card, doubling everything and use the battery to recharge the card.

If you set up an infinite jera combo in The Chest, you can copy gold chests, which contain items.

D6 lets you to reroll the 4 gold chest items in the first room, unless you pick them up. To speed up this you should get 9 Volt (which reduces the charge of any Activated Items by 1) and the Habit (when Isaac is hit, 1 point of an item's charge is filled).

A way to break the game is to get Undefined, which teleports Isaac to either the Secret Room, Super Secret Room, Treasure Room, or I AM ERROR.

If you get to the Chest with it you can repeatedly reset the floor by going to the I Am Error room, getting more and more items and making your Isaac the most broken character ever made, with easily over five times the amount of items you'd get in a single run!

I found a breaking seed which makes it easy to break the game. The seed is DP4F HHZ2. On Basement XL, there is a jera rune in a golden chest next to an item room. On depths XL, there is a blank card in the shop. The best part about this seed is the Chest. Right of the starting room is a 48 Hour Energy Pill. To the left is a regular chest. Unlike a lot of strategies, this one isn't flawed.