• Make 3 blocks of rotten flesh by filling a crafting table with rotten flesh
• Make a corpse head out of rotten flesh and a skeleton skull
• Make a chest cavity out of rotten flesh
• Line up the flesh blocks and the chest cavity in a T shape
• Put an item inside the chest cavity(or don't)
• Use shears to sew it up
• Place the corpse skull on top of your finished structure
• Strike the skull with lightning

And then, you'll have a monster of your own creation

Blaze Powder - Blazing Abomination

Bundle - Storage Abomination

Copper Ingot - Electric Abomination

Eye of Ender - Cyclops Abomination

Gunpowder - Explosive Abomination

Saddle - Hunchback Abomination

Rotten Flesh - Rotten Abomination

Nothing - Abomination

Abomination - Neutral, Made by sewing up an empty chest cavity, just a normal frankenstein's monster-esque guy, gives the achievement "It's alive!" when created.

Blazing Abomination - Hostile, Made by putting blaze powder in an empty chest cavity, works like a stationary summoner, when the player isn't in attacking range, it gets inflated, and when its hit while inflated it deflates and spawns 3 small creatures made of fire called flamelings that set the player on fire, also sets the player on fire when it attacks.

Storage Abomination - Neutral, the most friendly of the abominations, Made by putting a bundle in an empty chest cavity, works like a walking storage, it follows the player who summoned it around and you can access 27 inventory slots by right-clicking it, drops a bundle on death.

Electric Abomination - Neutral, Made by putting a copper ingot in an empty chest cavity, when attacked, summons a lightning bolt to its location when the player is near, drops a lightning rod on death.

Cyclops Abomination - Neutral, Made by putting an eye of ender in an empty chest cavity, has one eye and looks corrupted by the void, is angered by looking it in the eye or attacking it, when attacking it has a chance to teleport the player randomly, it also teleports itself when attacked, drops an eye of ender on death, when attacked gives the advancement "My name is nobody" inspired by homer's odyssey

Explosive Abomination - Neutral, Made by putting gunpowder in an empty chest cavity, when attacking has a chance to make a small explosion, also explodes on death.

Hunchback Abomination - Neutral, Made by putting a saddle on an empty chest cavity, can be used as a relatively fast mount because it has a built in saddle, negates damage from the player when ridden, you can press R to make it do a charge attack where it dashes in front of it and deals damage to mobs it hits, drops saddle on death.

Rotten/Hivemind Abomination - Hostile, Made by putting rotten flesh in an empty chest cavity, looks like a hivemind, mutated and has multiple limbs and heads, attacks anything that gets near it and deals massive damage, grants the achievement "This is a crime against humanity" when summoned, drops rotten flesh