You judge all horses, regardless of age or gender, on the same four categories:
In that order.
Quality means the eye-appeal of the horse, specifically head and neck and how it all ties in together.
You can find a lot of judging resources through universities with strong agriculture programs or that are linked to the state 4-H program. Start googling.
Here is one to get you started: http://www.cals.ncsu.edu/an_sci/exte...ing_Manual.pdf