Rules for creating thangkas?

In [this article](http://himalayancollections.commons.yale.edu/exhibits/show/economic-and-urban-development/commercial-tourism-and-thangka) the author discusses the problems of the commercialization of creating thangkas. Nowadays thangkas are created quickly and sold to tourists in shops in the streets of Lhasa. I was wondering, what exactly makes a Tibetan Buddhist painting a genuine thangka? I know that traditionally thangkas are made according to a ritualistic process and have to follow strict iconography rules. But what exactly is this process? And where can I find information about these rules? In the article the author gives an example of a iconography rule that apparently is formalized somewhere: > Buddhist texts dictate that the Ghanta is at the left hip and the Dorje in the right hand. I know there are books available on this (e.g. [this one](http://www.amazon.com/Tibetan-Thangka-Painting-Methods-Materials/dp/1559392576)) . However, I'm not sure if the information in there is correct and complete. I'd prefer more original sources (translated scriptures) if that's possible.