As a fellow noob, can only share my experience:
1)  There are no walkthroughs/guides
2)  You will read and be told it's incredibly complex and difficult and time-consuming (which it is!)
3)  Whether starting from scratch or not, there should be some pre-existing bridges (meshes) out there in different formats which you can load up to get a basic idea of dimensions/size.   Rest is down to your modelling skills  

4)  Fed bridges are probably easier to do and more popular
5)  err... that's it. 
Hope been some use.
fellow beginner (WIP)