So, check out this mob trap I've built. I've got skeletons there drowning and, as they die, they're dropping an infinite source of arrows and bones, which flow to this collection point and I can pick right up. Let's go check it out. So, there's this three block deep layer of water and the skeletons fall in here and drown. They can't fit underneath there but their items can. Once they die, they flow out and flow straight to this collection point.
Let's go check out the actual spawner in the other room. Oh wait, let's wait for this guy to die. Nice. Right, so what I'm actually going to do is turn on peaceful mode. This way, I can show you the spawning room and I won't be bothered. So, right through this wall is a mob spawner. See right there, you'll find these blocks in dungeons along with chests with loot in them. You can incapacitate them by lighting the room with torches or by turning on peaceful mode but just make sure when the time comes to turn your mob spawner on that these torches are all gone. You can see a little skeleton in there spinning around. That means that this spawner spawns only skeletons.
Let's take a look down here. There's a row of water in the back here and that pushes skeletons towards this drop. They fall in and start to drown, just like I am right now. Okay, let's go find a dungeon and build a mob trap from scratch. So, here's a dungeon and a skeleton spawner. You can see them spinning in there. Let's go over what items we're going to need to build this mob trap. Plenty of cobblestone, some ladders to get out when you're done, two buckets of water to make an infinite spring, plenty of torches, some glass, some dirt and finally a sign. I'm using these torches here to incapacitate the mob spawner.
What I'm going to do is place them on dirt instead of cobblestone. Replace that with dirt, put the torch back on. I need my spawning room to be five blocks wide, which this one already is and it needs to be seven blocks long, which this room already is as well, so it's perfect. I'll need some cobblestone to seal it up so the skeletons can't get out and then I'm going to dig down two blocks so I can actually walk underneath the mob spawner. Destroy all these blocks but leave the two dirt blocks that you placed torches on. Those are what are incapacitating the spawner right now.
Once you've done that, you need to decide what direction your water will travel in. I'm going to have it go this way. So, I'm going to dig down three blocks at the opposite end of the room, all the way across the entire width of the room. I'm going to leave myself some steps so I can get out of here though.
All right, now once I've done that, I'm going to dig out this secondary room. This will be the room where I collect items and it needs to be at least seven blocks deep.
So there you go. I've created the room that's two blocks high and seven blocks deep. I'm going to place glass all against this back wall. It isn't necessary to use glass. You can use any block but I like glass so I can see all those skeletons dying. I'm going to dig this ceiling out just a little bit more, again, not necessary but it's nice.
I'll head back up into the first room and now it's time to start using the water. I'm going to dig into this wall here and make myself an eternal spring, I'll dig back three blocks and those last two, I'll dig down. Pour out my water buckets and place water in two opposite corners. Now I can draw an infinite amount of water from here.
I can actually get rid of these steps now, won't be needing them and just line the back wall here with water. One there and there, refill and then two more in the opposite corners and that should be enough. You can see all those blocks are sources of water pushing in this direction. Now, refill at least one more bucket. You're going to need it later.
Cover up your infinite spring and break these torches on the side of the wall and now, because I've left these torches on dirt, I can break them really quickly with my shovel and get out of here before the skeletons arrive. There they are. Pull out my glass and just close up this last whole and my trap is ready to go.
You can see them dying in the distance there and dropping some bones and some arrows. Now, because the water travels seven blocks and this room is seven blocks deep, I'm actually going to make it a little bit deeper just by one more layer. So, my final dimensions are 2x5x8.
What I'm building here now is a collection trough, so I can get all these items to go to one single spot. Take that last bucket of water, place it there. I'll dig this out a ways as well to that seventh block. The water will stop flowing and this is the magic spot where all those arrows and bones will arrive.
Great. Now, to get out of here, I just dig straight up and place ladders all the way to the surface. This is a quick and easy way to get out of here but it also makes it a quick and easy way to come back down to pick up all these drops. I dig out a hole that's 2x1. That way, there's room for both my torches and my ladders.
And there it is, the surface. I made it. Once you get out, the first thing you're going to want to do is to place a sign so you know later on where this path leads to and I can head down quickly and grab my drops. Because this is wide enough, I can actually start to fall much faster but catch myself on the ladder. Be careful, if you fall all the way, you will die. There they are, plenty of bones and plenty of arrows. There you go. That's how you can build a mob trap around a spawner that's spawning skeletons and zombies.