Google has released the full version of its Search Quality Raters guidelines & handbook. It’s 160 pages of SEO insight into how Google rates the search results they are testing. So it is not a definitive ‘how to’ guide, but it’s one of the only information sources you will get hold of.
Google’s Mimi Underwood said that “ratings from evaluators do not determine individual site rankings, but are used help us understand our experiments.” And, “The evaluators base their ratings on guidelines we give them; the guidelines reflect what Google thinks search users want.”
There is an in-depth read on this document available on the SEM Post here.