First, you'll need 2 things: The new maps .scn file and the map's terrain code.
You place the .scn file into the ScnObj subfolder in your Napoleonic Wars module on your server, overwriting the previous one there.
Next you open the scenes.txt file in the NW folder, and hit Ctrl+F. Type in part of the map name that you just overwrote (For instance, 'arabian' if you're looking for Arabian Village). It will take you down to that in the file. Note that if you ARE replacing Arabian Village, there are multiple maps that begin with that word. You will need to find the specific one you're looking for.
It will look something like:
scn_mp_arabian_village mp_arabian_village 256 none none 0.000000 0.000000 100.000000 100.000000 -100.000000 0x000000012002a0b20004992700006e54000007fe00001fd2
0
0
outer_terrain_steppe
You replace the bolded text with the terrain code that you have for the custom map.
Now assuming that you've done all of this correct, when your server switches to this map (In this example Arabian Village), everyone on it will download your custom map and load it instead.