start by downloading the code from the custom maps mod, then add your map to it (create a new data class and modify main.cs). for now, use placeholder data for the tracks.

now install this mod, get into that new custom map you added and then use F1 or F2 and click on the map to generate code (it will be in map.txt)

F1 generates tracks by clicking.

F2 generates areas by drawing polygons, before you use it press keys 0,1,2 or 3 to select the type of area. Polygons autocomplete themselves. Look at the console to see which type you are drawing.

When you're done put that generated code in the data class that you created in the custom maps mod. You can also make a pull request if you want your map to be included in the public custom maps mod.

if this is too complicated for you, wait until bowdown releases a proper map editor.