perhaps your paper trail
receipts might support an opinion
a paid fine could support an opinion about the law
or an opinion that crime doesn't pay

signed affidavits from yourself and witnesses might be another way along with many other kinds of proof used in a court of law

you do that every day (except writing it -- unless you keep a diary). when you make a choice whether to eat something that has been sitting in the fridge for a week and smells like a dogs butt , or choose not too. that's having an opinion based on past experience (an old case of food poisoning ). Telling someone to go to the doctor who is coughing up green gunk and can't breath is another expression of your opinion based on a friend or yourself eliciting the same symptoms. We all do critical thinking every day of our lives, it is just that some events are a little more serious and critical than others. But daily usage makes us more practiced and able to perform it better and better! good luck in your class.