Unlocked by getting 4 Cube of Meat in a single run. Each one you get will "level up" your meat cube, starting as a regular orbital that blocks shots and does contact damage, level 2 is the same thing but it also shoots tears, level 3 turns it into an independent follower that runs after enemies and deals bigger contact damage, and level 4 is the same but grows larger and deals even more damage. There a a few ways to do this.
The Potato Peeler lets you give up a red heart container for a Cube of Meat. If you have 4 red hearts or more, you can use it 4 times and get the achievement right away.
The Box of Friends lets you duplicate followers for the room you are in. Each time you "duplicate" the Cube of Meat, you actually level it up. So if you have at least one Cube of Meat and some batteries on the floor, you can repeatedly use the Box of friends to get it up to level 4.
And the main method for obtaining them is beating the Harbinger bosses, the first one you beat drops either a Cube of Meat or a Ball of Bandages, and every one afterwards for the current run will drop the same item. So if Famine drops a Ball of Bandages, you'll have to reset and try again.
Famine appears in chapter 1
Pestilence appears in chapter 2
War appears in chapter 3
Death appears in chapter 4
Conquest also appears in chapter 4, but is unlocked only after taking 10 items from Angel Rooms. Conquest will ONLY drop the White Pony, so if he appears as your 4th Harbinger, you'll have to fall back on either the Potato Peeler or Box of Friends to get your 4th Cube of Meat.
Using the Book of Revelations item greatly increases the chances of a Harbinger appearing as the boss for that floor, except they will not replace Mom or Mom's Heart/It Lives.