The first element of postmodernism that was found in "Slaughterhouse 5" was that it was humorous. Kurt Vonnegut would say a lot of funny things that you wouldn't get unless you actually thought about it for a while. He said many things that required no thinking at all and that is what made them funny, because the reader would think that he meant something entirely different but really he didn't. When you read "Slaughterhouse 5" and actually try to understand it can be easy to see the humorous element of postmodernism in it.
The second element of postmodernism that was found in "Slaughterhouse 5" was that it was not in chronological order. Vonnegut would jump around in his book from one year to the next, and he made it your job to understand where and and what point in time he was in. That can make a little confusing but that's what postmodernism is all about and without this, this book would just be a normal modernists book. Kurt Vonnegut did a really good job when he jumped around from year to year in his book.
The third element of postmodernism that was found in "Slaughterhouse 5" was that it broke the 4th wall. Kurt Vonnegut actually put himself into the book and that was totally different from the modernist books that were written before him. When he put himself into the book it was like when you are watching tv and the character on the show turns to the camera and looks at the person watching the show and starts to speak to you. This wasn't done before and that makes it postmodern more than any other thing.
So those are some things that can help relate postmodernism to "Slaughterhouse 5". And again they are 1. humorous 2. its not in chronological order 3. and it breaks the 4th wall.