At the bottom right of the post you want to quote, there are three buttons, the one on the left is reply with quote. Hit that one and the post you want to quote will appear in the reply box.
If you want to quote multiple posts, the middle button is multiple quote. Just hit that on each post you want to quote, then hit post reply, and you'll see all the posts in the reply box for you to comment on individually.
Good morning, son
In twenty years from now
Maybe we'll both sit down and have a few beers
And I can tell you 'bout today
And how I picked you up and everything changed
It was pain
Sunny days and rain
I knew you'd feel the same things...