Okay, here is an as easy as it gets explanation. Unfortunately, it's not complete yet (read the bottom note):
Preparations
- Go into your M&B installation folder and in there, find the subfolder "Modules"
- Make a copy of the "Native" folder and paste it into the "Modules" subfolder with a new name, e.g. "Mapping"
- Start M&B and in the launcher, click on "Configure", go to "Advanced" tab, and check "Enabled edit mode"
Creating a terrain string
- Start M&B and in the launcher under "Current module", select your newly created module
- Start a new campaign, and when you are on the world map, click on the new button "Terrain" in the lower left
- Play around with the options (note that vegetation, etc. placed this way can't be removed later) and once satisfied, click on "Copy" to copy your terrain string to the clipboard
- Paste your terrain string into an editor to save it somewhere
- Exit M&B
Editing strings.txt
- Open strings.txt in your new module folder and find the string "multi_scene_end"
- Directly above this line, insert a new line with the strings "str_example_map" and "My_Cool_Example_Map", both without quotes and separated with a space
- Scroll back up to the top of the file and increase the number on line 2 by 1
Editing scenes.txt
- Open scenes.txt in your new module folder and find the string "multiplayer_maps_end"
- Right above this line, insert the following text (note the correlation with your entries in strings.txt):
scn_example_map example_map 256 none none 0.000000 0.000000 100.000000 100.000000 -100.000000 0x00000002200005000005f57b00005885000046bd00006d9c
0
0
outer_terrain_plain
- Replace the long string 0x000000...etc. with your terrain string you created earlier
- Replace "outer_terrain_plain" with one of the following: outer_terrain_beach, outer_terrain_desert, outer_terrain_plain, outer_terrain_snow, outer_terrain_steppe
- Scroll back up to the top of the file and increase the number on line 2 by 1
Editing scripts.txt
- Open scripts.txt in your new module folder and find the string "multiplayer_fill_map"
- Right below this line, you will see a loooooooong line of numbers
- Despair
Hosting and editing the map
- Host a new multiplayer map of the desired game type and select your map
- Hit Alt + Enter to enter windowed mode
- Hit Ctrl + E to start Edit mode (sometimes takes a little while to load)
- When you're done, hit Ctrl + E again and when prompted, save or discard the changes
As you can see, the instructions are incomplete. The reason is, I'm using the module system, which does that step automatically for me, so I don't know what numbers to add where manually. This line tells M&B what maps are available in what MP modes. You won't be able to host your map without this info. You COULD load it with the quick scene chooser though, but getting that to work is also quite bothersome for many ppl. Maybe someone else can complete this step. If not, I might get back to it later...