[DE+EN] [OMSI] Download-Links Doku, SDK, SDK-Tools, DIN-Fonts - Allgemeiner Objektbau / Object Design - Marcels OMSI-Forum
The "Inofficial OMSI SDK" (originally for Omsi 1, but applies to Omsi 2 too) contains a useful series of tutorials on using the Editor, as well as some more complex tutorials on creation of buildings and junctions in Blender. I believe there are also some YouTube tutorials available, but as I have never used any of them I cannot say how useful they might be. With some basic knowledge at your disposal, some of the best ways to learn techniques is to look at what other people have done - either by watching mapping streams to see how it's done in real-time, or by looking at an existing map and "dissecting" it to see what's been used to create an effect - and also to just experiment. If you create a test map that you don't mind breaking, you can practise modelling little scenes and see what does and doesn't work. In many cases there are several different methods to do the same thing, with each having their merits and mars. If you're looking to make something more realistic, the best advice I can give is to look at the real thing and see what's done and try and recreate what exists in real life, rather than what you think exists; most situations have been encountered by someone in reality too.
Basically use OMSI map editor. Can't remember how you open it for the first time as that was ages ago (as now have a shortcut to it instead) but in the OMSI directory there's a PDF called 'manual SDK' and it tells you how to build a map using the editor.
Well that's what it's like in OMSI1 anyway.