How do you quote parts of a post?

I have seen people do this and I am not sure how? It is helpful to respond to longer posts and I am still oblivious as to how this works and would like to know.

Sorry if I am not supposed to put this thread here but I was unsure where to ask....

Anyone?

Do you mean like this?

If so, just hit quote then double click on the message and delete the portion you want to get rid of.