1. Create a line of a chosen building material (preferably cobblestone due to its common occurrence and resistance of ghast's fire balls) from one end of the marked area to the other.
2. Place pressure pads on the top of the line and redstone torches on one side of the line.
3. Place a line of cobblestone on top of the redstone torches and then place another line of redstone torches on the outward side of that line.
4. Place a third line of cobblestone on top of the second line and then place a line of sticky pistons that will push those above the first line of cobblestone.
5. Place half slabs on top of the third line of cobblestone to prevent wither skeletons form spawning on top of the collectors instead of inside them.
磨床
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.
其它補充
You may want to add a cage of some sort around the farm to prevent ghasts from blowing up the structure during collection. As well as that, most wither skeletons only drop coal and bones and only when they are killed by a player is a wither skeleton skull dropped.