Zealot

I need help I have the two main lines done but! I have 5 spurs to do I want one spur to go around on side of a mountian to a mine. Do I start building the mountian then put the track on it or make risers for line and put the track up then build the mountian. This spur should also run over a tunnel for the two main lines. Also I want a small wooden bridge over a creek need to know when to build it. It is on a 4x8' ho layout.