Well first take care to know what you're getting into. Cops who rat out other cops get death threats because the police are a mafia. I'd get in touch with a whistleblower organization of your choice and ask for advise, but the basics would be wear a tiny hidden camera and try to record training and anything illegal. In most states quotas, for instance, are illegal per se, but it's not enforced because the cops would have to arrest themselves. A number of departments have been brought down because people simply recorded their 'captains' saying "hey, I need you to be finding more ways to fine people" Another approach would be to show what the cops thought (if they were the mythical 'good' cop) and record them saying "Damn the mayor won't let us arrest these blm bastards who burn loot murder, but tell us to arrest maga hat wearing people; it's all political" I'm sure that kind of conversation happens every single day amongst the 5% of not shitty cops.