This achievement is not difficult, but incredibly time consuming, the more people you have helping you out, the faster this will go (you all will get the achievement).
The achievement requires you construct 9 maps and fully explore each one with a different, adjacent part of the world.
Constructing a mapA map is constructed using 8 pieces of paper and a single compass in a crafting table. Paper is easily obtained using 6 pieces of sugar cane, and a compass is built by using one piece of redstone and 4 pieces of iron. Because you need a minimum of 9 maps to complete this, you will need 9 compasses and 56 pieces of paper.
It should noted, if you can't be bothered building all these maps, everytime a new player spawns into the world, they are given a free map, so you can just keep spawning in guests and having them place their maps in a chest to do this much faster.
You can start on this as soon as you have 1 map, but it will be easier if you just wait until all 9 are built. From there its really just a process of filling up each map fully.
Edit: I highly recommend you be on peaceful doing this, I can confirm that if you are on any difficulty and switch the difficulty on your world to peaceful achievements will not be affected. While on peaceful you do not lost food and there will be no unfriendly mobs.Edit 2: I also suggest you turn off your HUD in settings. The UI elements will block parts off parts of the map, which can make it difficult to see areas of the map you have missed in these areas.Filling up the mapsI would recommend the area you have a house set up in be the middle map, so take a single map out when you are by your base and press
to open it up. Doing so will reveal a small circle of terrain around you (an arrow) as well as show any other players within the map as different colored arrows.
Walking in any direction will reveal more of the map, so needless to say, you need to fill up the entire map. Try and systematically move through the terrain, so as to not miss anything, go back and fourth. It is quite tedious to do this by just walking, as such I recommend you try and find a horse to ride (if you don't have one already, they will spawn in grassland biomes) as well as bringing a boat, allowing you to cross bodies of water much faster. Fill up this entire map and look carefully at it to make sure there is no bits you missed,
Once you finish the first map, you want to use it as a guide to see where to start the second one. Since this one is the center most map, you can go either up, down, left or right of this map to start the next one. You want to watch yourself on this map and as you approach the border of the map you will see you go from an arrow to a circle, this means you have entered the next map territory and can open a fresh one, beginning anew.
It is important you do not open up a new map in a territory already explored, as this will immediately fill up the whole map with whatever you had already explored in that area, so you have a duplicate map, and it can not be reset, which means you will need to build an entirely new one.
From here, the process is simple, just fill up all nine maps, it takes a very long time, but be deliberate so you do not miss any spots. Getting additional people in your game can speed things along quite a bit, everyone taking there own map and filling it up, make sure you coordinate well so you do not fill two maps with the same area.
Once all 9 maps have been built, you will require 9 item frames to put them. An item frame requires 8 sticks and a single piece of leather. Place all 9 in a 3x3 square on any wall and fill each item frame with the correct map (so the ones adjacent to each other in the world are also placed in adjacent item frames.
When they are all placed you get a really cool huge map made up of all the smaller maps, the player who places the final map gets the achievement, but if you just punch any map out and have another player place it they too will get the achievement.
If you place the final map and it doesn't unlock, make sure the maps are in the correct order, the terrain will line up if they are. If this doesn't work, chances are you misses some part of the terrain on one map, so study each one carefully. The color of unexplored map is very similar to sand, so make sure it is actually missing, and not just sand.
This achievement is very boring and takes a long time, but you get a nice 40 GS award for doing it, as well as an awesome map to put in your house. Good luck!