When you have the impersonal "you", you use "se" before the verb and you conjugate it for the 3rd singular person. In this case "como se cozinha peixe?" (Cozinha does not work only for the imperative).
For "you" not impersonal, the best is to include the pronoun: como você cozinha peixe?
Cozinho = i cook. Cozinha= he/she/you(sg) cook. But if it is he or she youd add ele or ela for emphasis. So you would assume you(sg). Its my understanding (and i could be extremely wrong) in general, assume you. By adding vocé, one is emphasizing YOU. For example, if i said "how do you cook fish (como cozinha peixe) it could mean "how do you (in general) cook fish. But if you said " como vocé cozinha peixe" then it means "how do you (and only you) cook fish". Dont quote me. My native language is english. Buts thats how I use it.