"Monster foci are used in the seer stone, and instruct it to search for nearby mobs (Zombies, Skeletons, ect)."
The monster foci are only used in the seer stone, and give the seer stone the function to search for any mob like the ender dragon or even silver fish.
To craft the monster focus, you need 2 iron swords and a standard focus. Place a standard focus on top of an iron sword then an iron sword on top of the standard focus. This crafting recipe is semi-universal (able to go in any of the slots, as long as the correct format is still in palace)