Instructions
Build a wall that measures at least 25 block long, 25 blocks wide and one block high. Mobs only spawn if you are at least 24 blocks away, so this leaves enough room for the mobs to spawn if you stand in the middle of the space.
Light torches every five blocks around the perimeter of the wall. This prevents mobs from spawning outside of the structure instead of inside.
Go back into the structure and wait for a mob to spawn. Some kinds of mobs, such zombies, creepers and Endermen only spawn at night.