To make a map you need a compass (iron + redstone) and some paper. If you still have the map you spawned into the game with, and you are still in that chunk of the world you first opened it with, that will be the central map in the 3x3 square. If you don't have that map/are in a new area, don't worry about it. You can just start with a new map. Don't open a new map when you already have one for that area. It will just make a duplicate which is no good.
With your 9 maps, we can begin exploring. I like to knock out an entire map before I move onto a new one. It is also helpful to store your maps in a 3x3 square in your inventory. Put your world on peaceful so you aren't wasting food. Bring a bed to avoid the night. Have boats and stick to the water as much as you can. If available, make some swiftness potions. When exploring, start by doing the outline first and filling in the center after. Make sure you fill in all of a corner. This is the easiest part to mess up because though a map might look completely explored, there could be one square missing in a corner, and it is a pain to try to fix.
Once an entire map is explored, head to the next area. For example, if you start with the central map, head north so we can explore the top-middle map. When you pass the edge of where your map covers, your colored arrow will turn into a circle. This means you can pull out a new map and open. Explore this area and head to the next one until you have 9 fully explored maps.
Finally, on a 3x3 section of wall place 9 item frames, and put the maps in the correct spots in these item frames. With any luck you'll have a giant ass map that serves little to no use other than looking cool. Oh, and you get an achievement.