In this Instructable, I will show you how to build a simple, starter mob farm.
Step 1: First Farm
Start by making a 26 high block pillar
Step 2: Channels
Make a cross with 7 blocks on each arm.
Step 3: Levels
Add the secondary spawn levels
Step 4: Spawning
Add the primary spawn level,the water and the signs.
Step 5: Walls
Add the foundations for the walls.
Step 6: Building Higher
Add the second level to the walls. Make sure they look like the ones in the photo.
Step 7: Taking Cover
Add the roof and you are done!