There are no specific tutorials (that I know of) to create parametric (stretchy) objects.
If you have a good understanding of how to script in GDL you shouldn't find it too hard.
If you are not familiar with GDL scripting then that is what you will need to learn.
There have been a few post here in the past listing good GDL resources to help you learn.
The GDL Cookbook by David Nicholson-Cole is still very good in my opinion (although a little dated now).
In fact it does have some examples of stretchy objects in 2D and 3D.