There aren't too kinds of grammars, prescriptive and descriptive. All grammar is descriptive. What we are interested in is what the rule of the language actually are, not what we would like the rules to be according to other extraneous criteria. Grammar is evidence based. The rules can be deduced from the practice of those using the language.
We can use the work of canonical, classical writers, because presumably they are not merely competent native speakers, but also beyond reproach. The Horatian concept of norma loquendi is also useful.
Let's say there is a dog. We have some mofidiers to describe the dog.
Everyone knows that you don't say "brown hunting big dog" or "hunting dog brown big." We don't need to prescribe the order of the adjectives, sit down a child and teach her to rules here, because nobody makes these mistakes.
So what is this so-called "prescriptive" grammar you've been hearing about? Mostly a combination of invented rules and proscriptions against dialectal forms outside of the norma loquendi of educated speakers. There is no prescriptive grammar of the language, since prescriptivists don't care about the actual grammatical rules of the language. They only care about a few rules that can be used to differentiate between the educated and those with less education, or between those who've read certain style manuals and those who have not. Prescriptive grammarians are simply incompetent sociolinguists, who don't even know that that's what they are.