This is a simple mob grinder that you can make most anywhere.
Step 1: Materials
Your materials can be anything that you wish but but these are some examples of what you can use.
Step 2: The Hole
Make a one block tower 24 blocks high and on the 24th block make a 3x3 hole. You must make it at least 24 blocks high of the grinder won't work. After the hole has been made you can destroy the one block tower below the hole.
Step 3: The Water Way
Including the block that makes the hole, the water way will be 9 blocks long and the same width as the hole. On the outside blocks add a small wall. It should look like the third picture and to treat that everything is right so far just add three buckets of water up against the wall and if it doesn't fall through the hole everything is perfect.
Step 4: The Mob Walk Way
Add an extra two blocks to the side of the small wall.
Step 5: Adding the Wall
Add a wall that is no more and no less than three blocks high. Make this wall all around the outside of the mob walk way.
Step 6: The Finishing Touches
Add a ceiling so that only two blocks show when you look inside. You can leave the same size hole in the ceiling so you can add more floors. After that add a tunnel from the bottom of the grinder so that it is 3 blocks in length.
Step 7: The Lights
You can add redstone torches to the inside as an option (adding redstone torches will not affect the spawn rate of the mobs). It is recommended to add lights to the top so that no mobs will spawn on top of the grinder which will affect the efficiency of the grinder. You should also add lights to the bottom so that bats won't spawn. Add some sort of lights to the ground below the grinder so that mobs won't spawn near you.