Minecraft wiki entry for Wither_skeleton_farming, associated text: After the mob collectors have been made correctly and wither skeletons are being spawned, you will need to make a bucket shape of cobblestone under the collector so that all spawned mobs are at one main point. This can easily be achieved using the remaining cobblestone by making a tube from the collectors to the area chosen for grinding. Once the mobs are at the grinding point they can all be killed in one area simplifying collection and speeding up the whole process.