It’s no difficult at all, you can use this tool, http://www.microdatagenerator.com/product-schema/
I have followed the “google rules” but, unfortunately, rich snippets are not showed. I went back with the original code delivered by the guys from Rich Reviews and it is working until now.
Same issue for me but I can say that google will show correctly the overall rating even though google shows “error” on the testing tool.
