The first step to proving a statement indirectly is to know and to state all possibilities. For example, if a soccer match is scheduled on Saturday and you show up and no one is on the field, instead of definitively stating, "I have the incorrect date," you would state all the possible reasons why no one is on the field. This could include the possibility that everyone else got the date wrong, everyone else experienced amnesia about the event, everyone got sick and everyone died in an accident.
Next, you must assume the opposite of the statement you want to prove. So, in the soccer match analogy, you would assume that there is no way you got the date wrong. Assuming the opposite of what you are trying to prove logically leads you to begin examining all the other possible reasons for why no one is at the game.
The next step requires you to reason through the other possibilities using deductive reasoning, which arrives at a conclusion based on accepted premises. In the soccer match example, you could take each reason and apply deductive logic to test the soundness of your premise. For example, you could state one argument this way:
1. If they had remembered a match was scheduled, everyone would have shown up.
2. No one showed up.
3. Therefore, everyone must have forgotten.
While this is a valid argument, it is based on a false first premise because there are many other reasons why people might not have showed up. By going through each premise in this way, you will reach a contradiction.
Using the soccer match example, it's clear that every conclusion you draw about the other reasons for no one being at the match other than that you got the date wrong will result in a contradiction. It's highly unlikely that every single member of both teams forgot the match. It's also highly unlikely that everyone died in an accident on the same date or that they experienced collective amnesia. And if no one told the teams they were playing, how did you obtain the information? Therefore the assumptions that are the opposite of your statement "I must have gotten the date wrong" are rendered invalid. Even if both teams were given the wrong date, that still proves your statement that you have incorrect information about the date the game is being played. In a situation in which only two possibilities exist, this method of indirect proof is much easier.