Hello!
I'm not a specialist in matter of creating routes - I try to understand this since several days. However, I can answer you a bit...
1. First you need to have an idea, a plan, a design, how it should look like. You can paint it with pencil or use a real map. You must exactly know what you want to get. This is the most important part.
2. Some people use 3D graphics softwere like 3dmax or gmax to create the vectors part of scenery. I'm not deep in this, I don't know details. With this softare you can enter routes, render previews and correct details.
3. Then, some special scripts are used to geneate the .scn files that are used by the simulator. There's no a way back. From this point, all other corrections are done in Notepad or other text editor. You must know how the commands are built, what is where in files. All the events are written manually.
4. To view the results, you can use the Szopa Track Viewer, also called STV. It shows tracks, tractions and many parametrers, but not all. It cannot be used for editing - you edit files with text editor and reload data to STV.
5. Beside the vectors and events, you need also models of buildings, tranis, cars, etc. and textures for them. You need to make a lot of photos and convert them into usable textures.
6. There's a lot of corrections needed. Usualy it takes half year to several years to make something sensible. You need to put a lot of time and work. Many people who start a own route, did not finish it due to diffuculty of the process.
I hope this helps. If you have a good idea, some spare years and you really want to learn a lot and do own route against all difficulties - please do it. Oterwise - forget it at the start.