You will need the OMSI_Editor but before that find
m-r-software -- Omsi folder and then go to Templates.
copy over NewMap file into Maps
rename the NewMap to 'My Map' or whatever you want to call it
then go into the folder and find Global
open in notepad
then edit the name and friendly name to 'My Map' and write a description the map if you so wish.
if you already have an image for the map, rename it 'picture' and copy it into the 'My Map' folder
then go into OMSI-Editor
don't load the 'where you were last time', press cancel and load navigate to My Map and open up the global file
You will then see a lovely green square surrounded by blue skies. This is, of course, your map...and feel free to do as you wish!