Making ethical decisions can be difficult.
The difficulty increases when you are facing a real decision.
So practice making these decisions when you don’t really have to make them. Pretend that the cashier gave you change for $20 when you handed him a $10 bill. Pretend that you overheard your aunt make a racist joke at the family gathering. Pretend that your boss turns a blind eye to your colleague’s expense fraud because your colleague is the top salesperson.
These or similar situations will happen to you. Practice them in your head. Decide now what action you will take. Note that I am not asking you to decide what the right thing to do is. You already know what to do (return the extra change, tell your aunt that her joke is inappropriate, report the fraud). The real difficulty is in deciding how to do it. It’s difficult even when it’s not for real.
So practice for the real game.