At the bottom of each post, you'll see a button that looks like this, .
When you click that button it will change to a button, that looks like this, .
For every post you want to quote, click the button, changing it to the button.
When you have the ones you want to quote selected, click the button.
This will bring you to the page where you compose your replies to posts.
You'll see all the posts you selected for quoting, already entered.
Just delete the part of the post that you don't want to quote.
Add your replies underneath the quote you're replying to.
When done, click the button, unless you want to see what it looks like before posting, then click the button.

Be careful when deleting parts of the quoted text, to not delete the quote tags.

[Used with the kind permission of TG1911 at BleepingComputer]