First compare it to a steel man argument.

With a steel man argument you make your "opponent's" argument as strong and perfect as possible. You help them make their case... and then you try to show them they're wrong while debating their powerful reasons.

​

With a straw man argument you do the opposite. You take the weakest, least useful part of their argument, pretend it's their whole point, and then show them it's wrong.

"I think you're a terrible boyfriend because you killed your girlfriend, you stole all her money, you set her family on fire and even after fifteen years in jail you still haven't said 'sorry'."

"Oh. So you think people are 'bad' boyfriends just because they don't apologise for every little thing? 'Oh I'm sorry for breathing loudly. Oh I'm sorry for eating three cookies instead of two'. You think a man should be weak and surrender to the woman like a slave? Well I think that's pathetic. And I'm glad to be more manly than that."