Instructions
Download the mod Craftable Spawners from a "Minecraft" resource site, such as Planet Minecraft, Game Front or Minecraft Forum.
Unpack the Craftable Spawners download file. Open your "Minecraft" installation directory and place the files into the "Mods" folder in the directory.
Load "Minecraft" and enter a single player game or load your own server.
Make a Monster Spawner block by place four iron bars, four moss-covered granite blocks and a final resource in the crafting grid. You will craft a different Monster Spawner depending on the final resource added. For example, an arrow will give you a ghost, a bone will make a skeleton and string will give you a spider spawner.
Place the crafted Monster Spawner on your inventory bar and place it down on the ground. It will continuously spawn a monster of the chosen type.