How can we inform the leader that he was wrong with one or more of his decisions ?

Some managers do not like criticism, especially from their employees. So you should ban And find a way in the right time without embarrassment if he loved the messages , for example, or after you complete  certain work .

Why to reach to that point with the manager things, from the bigining  things should be discussed and your point of view should be shared with him 

Full Agree with answer given by mr. Hamada

By making he or she aware that their decision may have been wrong. Having back up/support evidence to show and prove it rather then telling the leader or labelling them wrong. Making he or she understand why they are wrong and what they can do to correct this. As a whole working together to try and resolve or correct the decision or decisions then telling them as an individual. unless you feel that approaching them as an individual and explaining your point would be more easier. Again, dependant on the situation this might vary.

Thank You for the invitation .. I would agree with the answers that really answered your question... Variety of correct info and opinions ...Nothing to add !

Thanks I amagree with my colleague’s answers by Mr. hamada he gives us good answer

Generally mistake acceptance is not easy for leaders. When informed about the same they even might get angry or might just avoid.

Hence you have to see that the leader is in good mood when you are telling him his mistake. Carefully use words so that it does not trigger anger. Also you can express in written format. 

Upfront. But one should bring up the leader's decision that is likely to be wrong. The leader should appreciate. Never blame the leader but leader's decision.

An effective leader is proactively vulnerable to gain the huge reward - TRUST among the followers. For this, the leader should become proactively vulnerable by choosing to expose own weaknesses as much as strengths, mistakes as much as triumphs - and shortcomings as muchs powerful qualities.

Good leaders are generally open to dialogue so communication should never be an issue.

It is one of very difficult situation honestly, even highly educated big leaders or top management, they don't like to listen  from their employees that they are wrong on decision making or any mistaken from their sides. 

We need to review the person very carefully before going and telling such matters, no one can predict the mind of someone else. time to time or situation to situation persons are different. Some of them are shouting for nothing, even after they heard such mistake then needless to say their reactions badly.

Take good precautions while dealing with such situation otherwise sometimes you will be fired or it will help to make other reasons to fire you out simply. Psychology could not predict human mind in a clear way.