Look at the top right of the post you want to quote, click "quote".
Alternatively there's a quote button in the formatting tools while writing the post (looks like a speech bubble), but that won't say who the quote is from unless you write it in.
For example
Written like [ quote=Armast] [ /quote] without the spaces
You can also just hit reply, scroll down to the list of posts underneath the text box and hit "insert quote" on the top right of each post.
