Choose a flat area over an ocean, river, or high above ground to reduce nearby mob spawning
Gather materials:
Building blocks
Trapdoors
Slabs
Water buckets
Hoppers
Chests
Signs or fence gates
Cats
Campfires or magma blocks
Building scaffolding
Light sources
Build a spawning platform high enough to limit other mob spawns nearby
Make multiple dark spawning layers with solid blocks
Place trapdoors on the edges of spawning blocks to make creepers think they can walk forward
Add cats in the center or at specific points to scare creepers toward drop channels
Use water streams or funnel paths to move creepers into a kill chamber
Build a drop shaft tall enough to weaken creepers before they reach the bottom
Place campfires, magma blocks, or a manual kill area at the bottom
Put hoppers under the kill area to collect gunpowder and other drops
Lightproof the farm completely so no light enters the spawning spaces
Spawn-proof nearby caves, land, and structures within the mob spawning radius
AFK at the correct height and distance so mobs spawn only in the farm
Test the farm and adjust cat placement, lighting, and spawning platform spacing if needed