The nuts and bolts of any ethical decision-making process relates to doing the right thing. The difficulty arises when it's unclear what the right thing to do is. Any such scenario will involve identifying the ethical dilemma at hand and gathering as much information as possible. The process I would follow would therefore include gathering information, seeking advice from impartial outsiders and weighing up what the greater good in that scenario is.
To me, the greater good is important here. If you worked in a supermarket, and saw a colleague stealing food, the ethical decision is, almost without fail, to report your colleague's actions. However, if you saw a homeless woman with three children stealing a loaf of bread from your wealthy employer, the questions of ethics become far more complicated.
I would argue that the main factor to take into consideration in any ethical decision is common sense. Common sense will help you determine the scale of any misdemeanor. For example, if you work in a corporate office and saw a staff member stealing a ream of paper, it would be less relevant to report it than if you found someone stealing $100 bills from a cash register or petty cash.
If asked for guidance, I would suggest that a colleague who was required to make a difficult ethical decision should work out what the different possible courses of action are. Once all the options are on the table, it will be simpler to make the right decision.