Our friends at the World Intellectual Property Organization (WIPO) have recently conjured up a helpful new document, Making the Most of the Madrid System: Information and Practical Tips on How to Use Specific Forms.  It's a 61-page pdf which you can download here.  According to WIPO:

"It targets, in particular, small and medium-sized enterprises and users without much knowledge of the Madrid  System. This publication is divided in two parts: part I gives a brief description of the Madrid system and its main features, while part II has more practical focus on how to complete specific forms available for use in the Madrid system". 

Good news for non-English readers is that this publication will also be available in French, Spanish, Arabic, Chinese and Russian.