To create the .txt file you will need to first navigate and find your scenes.txt file which is found in the Module folder you used to edit the map. Once you have scenes.txt open find the name of the scene you used to edit yours with (for instance scn_my_scene_18), you need to copy the text following your scenes name and then put that into a new .txt document that you will bundle with your .sco file. The text should look like this but with your terrain values, also make sure to get the part that describes the terrain type for the outer_terrain or your map might look out of place.
scn_my_scene_18 my_scene_18 256 none none 0.000000 0.000000 100.000000 100.000000 -100.000000 0x0000000030000e0000080e030000650500001e1f00007ee2
0
0
outer_terrain_plain