Thiago Vidotto has produced a series of tutorials and guides for use in creating items for the Dota2 workshop. These are meant for the intermediate to advanced artist who would like to get some insight into some of the techniques and methods of the Dota2 pipeline. Each is fully laid out here on the site and you can even download PDF versions so you can review them even while offline.
Texturing Guide for 3D artists:
Composition Guide for 3D artists - This discusses ways in which help display and present your work