I wonder what 1:1 scale means, since Simutrans has no unified scale.
Regardless - I would start by choosing a map, or rather, two maps, if possible - one that shows roads and buildings, and one that shows the height of hills. Both of them would be placed in photoshop in one file, and adjusted so they exactly overlap. I would then resize them and choose a scale. For example, I could choose that each 5x5 pixel square corresponds to a tile in Simutrans (and use a grid).
Next step is to convert the map with heights into a proper heightmap for Simutrans. It's not too exact, so I would just paint it by hand in the required shades of grey, then scale it down (so one pixel equals one tile) and export it. This would be the basis for map creation.
Then, I would use the other map I still have in photoshop to determine what needs to go where, specifically planning out main streets, railways etc. - One important decision has to be made here regarding diagonals. What's more important, the 'straightness' of a way, or the angle? A straight street thats at an angle of, say, 30°, would look 'strange' in Simutrans, because it would look to be a lot of curves, not straight. I think it would look nicer to change the angle and have a straight way straight - and such things need to be planned beforehand and drawn into photoshop before placing it down.
Now I am aware of how to create objects for Simutrans, so I would probably continue by setting a custom pak together that fits my city and it's districts. One could just place every building manually, but I think I would use climates and clusters so they would spawn appropriatly when I place cities. The cities, of course, being districts. But that's just me, and honestly, I would not even start such a project, since five minutes in I would start pixelling one of the cities landmarks and realize a few hours later that that's NOT how you play, so don't listen to me
Oh yeah - don't forget to change the station radius to be appropriate for your scale, since most likely you would want to use the transport network of your city, not randomly placed busstops.