For this achievement you will need to find, activate and enter the End Portal. This is the easiest way I have found to do it and will not waste any extra Eye of Ender.
Items you will need in your inventory:
- 12 Eye of Ender (May need less but good to have 12 just incase.)
- 1 map
- 2+ shovels
- 2+ pickaxes
- 64 torches
- 1 sword
First, you will need to make a Nether Portal. Then on Peaceful enter the Nether and search the for a Blaze spawner. Once you find it, make a clear and covered path to the spawner from your portal (so no other enemies will bother you later on). Once done save your game, exit, and reload preferably in Easy. Equip some armor and a good sword and follow your path to the spawner. Kill enough Blaze mobs to get 6 Blaze rods then exit the Nether and turn your 6 Blaze rods into 12 Blaze Powder (Under food tab on the crafting table).
Next, you will need to get 12 Ender Pearls. These are obtained by killing Endermen. Wait for night and equip a good sword and some armour and head to an open grassy area (they are easier to see/kill there). Usually per night you can kill 1-4 Endermen and get about 1-2 Ender Pearls, so keep it up and eventually you will get 12, or have some friends help out. When you do have 12, take them and the 12 Blaze Powder and make 12 Eye of Enders (under Tools and Weapons tab).
Now save your game, exit and reload. Make sure AUTO SAVE is OFF. If it is on, disable it. Head outside with the 12 Eye of Enders and a map. Throw the Eye of Enders and follow it and retrieve them if they fall back. If they break, do not worry, as long as you DON’T SAVE your game, you will get them back. Keep following the thrown eyes until they hover above one area, this means that the End Portal is below that area. A good way to make sure is to throw them from different directions and make sure they fall back down in the same area.
Now that you found your spot, take out your map. Write down the X, Y and Z coordinates found at the top of your map and then EXIT GAME WITHOUT SAVING. Reload your save and you will be back with all 12 of your Eye of Ender (Note, if along the journey you break/lose all your eyes, mark the coordinates and exit without saving and go back to the last coordinates with all 12 eyes again and continue the search from there).
Next, pack up some shovels, pickaxes, torches and your 12 Eye of Ender and head to the final coordinates on your map. Once you’re there, save your game. Now dig straight down until you hit the monster strong hold (Auto save off, save before digging so if you hit lava or die you can reload from the top and avoid it next time). Search the strong hold killing the Mobs and planting lots of torches until you find the room with the End Portal in it. Place an Eye of Ender in each empty slot of the portal and it will start up. Save your game BEFORE you enter the End Portal, now hop in and the achievement will unlock.
EXTRA: It’s good to plant ladders on your way down to have a way back up your hole. Also, if planning on spending time in the End or going straight to “The End.” Achievement, it’s good to have a bow, armour, good sword, and a few 64’s of dirt before entering.