Advanced Mob Spawner: https://youtu.be/FMeR_xGWHVA
Mob Spawner Tips and Tricks: https://youtu.be/PO85rZ7KIlI
Mob Spawner EXP Farm Upgrade: https://youtu.be/mkFEsiSd9Hc
Skeleton Mob Spawner Exp Farm: https://youtu.be/88_AMSig1qo
As requested! Here is a tutorial on how to make a basic Mob Spawner/Mob Grinder.This Mob Spawner/Mob Grinder is the most efficient simple (no redstone) design that I have come across. This design can also be modified and used as an experience farm. Hope this helps you guys out in your survival worlds!