As per this post
I have located two monster spawners on Frontier by loading the map seed in a Creative single player server. I've only made these spawners accessible via the Nether and kept my "Nether Network" hidden from the rest of the nether system.
In the interest of fairness and balance I'm doing my best to show the entrance and paths to all areas I've connected via the Nether.
Spawn Portal in Nether
Firstly take the door to the path way down shown by the arrow.
Pathway from Spawn Portal
When you come down the path (stairs on the right) head down the corridor shown.
Hidden Entrance 1 (x:4 z:43)
Here is 1 of the hidden entrances (there are many more which you can find when inside the portal system) and the button to open the door is highlighted.
Open Entrance 1 (x:4 z:43)
There wasn't enough room to make the redstone circuit delay so the door opens and closes pretty quickly so it might take you a few attempt to get through. As a temporary solution, if you don't make it through the door in time it's been design to push you out the way rather than crush you if that's any consolation.
Now you're in!
I've tried to label everything as best as possible with signs pointing the way etc.
The first portal you see (right, x:-6 z:43) takes you to my underground place near spawn.
Continuing through the portal will take you to my Nether Wart Farm (x:-1 z:70, help yourself but replant please), Zombie Spawner 1 (x:-52 z:69) and 2 (x:118 z:46) and Skeleton Spawner 2 (x:5 z:42, near spawn).
Going left (but not back down the entrance stairs) will take you to a stronghold I found (x:85 z:30, with open portal to The End) and Arrow Spawner 1 (x:-28 z:-183) which is a long way away from spawn.
Zombie spawner 1 is complete but Zombie Spawner 2 (the other double spawner) is still under construction.
The completed spawner should leave Zombies with ~1/2 heart but it only spawns 1 zombie at a time so it's low yield.
The Arrow Farm near spawn has an automated grinder. There is a button near the entrance that *should* reset the circuit and a master switch where the arrows and bones float to that hold the grinder closed. I leave it closed when not using it because it's near spawn and doesn't need to be running all he time. You can also disable the spawner via a switch, something I also do when not using it.
The other Arrow Farm has a manual switch operated Grinder that needs to be left closed for them to collect above (and be far enough away from the spawner that they still spawn) and opened so the skeletons fall down and closed again to crush and kill them.
I've tried to leave instructions on all the walls but there is only so much space on the signs.
I might take some more screen shots of the other entrances, portals and nether wart farm later.
Hope this helps.