How do you tell someone that they are wrong without sounding like a douchebag or coming out as aggressive?

Related: How do you tell someone that they're wrong every time they open their mouth? Because I know more than a few people like this.

I think it's a matter of how you tell it. Get next to them so you don't have to talk loud. Tell them that you think that it doesn't go that way. Explain why you feel that way. Ask them if they think your explanation makes sense. Have an honest conversation.

Follow me: "you dumb ass, THIS is how you do it"

@C0D4 Do we get naked before of after saying that?

Find Sam Harris on YouTube... he does it all the time

Live in the Netherlands, honesty is appreciated. Not disagreeing with someone while you secretly do is more offensive than telling them No.

@Codex404 I had that environment in Germany too. I love it

In more complicated situations, if the other is minimally open, you can make informed questions, trying to understand the others thought process. This often results in a better understanding on both sides. People appreciate when they feel listened to and that somebody cares.

Don't focus on the fact that they were wrong, but more about how a "better" way to do it is <your way>. Explain the benefits of doing it your way instead of theirs, and point out (carefully) the flaws in their plan or logic. The idea is to educate, not belittle. If, no matter how hard you try, they still refuse to accept that they are wrong then the best thing to do is ignore them and let them learn the hard way. Just make it clear that if they do it their way, you aren't going to be cleaning up their mess

You proof that they are wrong, shame does the rest