For pretty much anything you might want or need to know about corsets,
this is a very informative site , that is easy to use.

Corset Gapping – By Lucy’s Corsets
 A very informative video on what the gapping in the back of a corset means. She explains what corsets are better, or more suited, for your body type and how to tighten them without ruining them over time. 

How to Lace a Corset – By Orchard Corset
A  video tutorial on how to correctly put the lacing into a corset.