WCF CustomBindings allow you to construct a binding to meet the requirements of your service, when one of the system provided bindings is not an exact match. For the most part, a CustomBinding will be built from the same “building blocks” (system-provided binding elements) used in the “standard” bindings. The real difference is that you get to decide which components are assembled together, rather than relying on the set provided by Microsoft.
You may find the WCF “BindingBox” useful to help you convert a standard bindng to a custom binding: http://webservices20.blogspot.com/2009/08/bindingbox-convert-wcf-bindings.html
Also, the following link provides additional information regarding CustomBindings: http://msdn.microsoft.com/en-us/library/aa347793(v=vs.110).aspx