There is lots of information out there about saddle fitting, some good, some not so good. Here are some vert useful guides and tools for understanding how to fit a saddle correctly
The passier saddle fitting guide is a very helpful short paper on how to measure and fit a saddle.
For a more detailed video on the more technical and complex aspects of fitting a saddle the video articles from Schelese Saddlery are quite good
(Here is the link as well in case it doesn't work http://youtu.be/U2mKz0uP_K8 )