My usual Order of Business:
1. Terrain with Transdem and overlay maps.
2. First attempt at laying track adjusting terrain as you go (particularly for the UK 90m DEM tends to be a bit averaged).
3. Roads and rivers while you still have the map overlay, making any further adjustments to terrain.
4. Global terrain texturing/painting.
5. 3D placement, buildings, foliage, stations etc. Fine adjustment to terrain texture.
6. Signalling, markers, interactives for stations and industries.
(This is an old MSTS habit and could probably be moved up the batting order, though it makes easier to do any track adjustment if you haven't got to delete the signals then replace them).
7. Fine detail - streetlamps, phone boxes, clutter, people etc.
8. Testing, testing, testing. Fixing, fixing, fixing. Ah, they won't spot that, will they?!!
9. Documentation then package up for distribution.