Found this hope it helps 😄
Just build four 8x8 platforms in a square with a 2-wide channel between each platform. put water in the channels that funnel towards the center. put a roof and walls over it. bam. mobs spawn on the platforms, they wander around, they fall in the water, water takes them to the center, they fall in a hole. if the fall is long enough, they die and drop their items. if the fall is not long enough they fall and just stand around. if the fall is just the right length, they will get injured to the point where you can just slap them dead.