It's really easy! Just put your mouse/cursor where you want to start, hold down the left mouse button and highlight the word, sentence or paragraph that you want to Quote. When you release the mouse button, the area will be highlighted with a choice of QUOTE or REPLY showing at the end of it.
Click on REPLY and it will move your highlighted words directly into the Reply Box and you can add your response.
Click on QUOTE and it will "hold" it until you are ready to reply in the Reply Box. You can quote as much as you want. When you are ready to write in the reply box, simply click on INSERT QUOTES (under the reply box to the left) and it will bring up all the areas that you highlighted, asked you if you want to quote all of these. Hit on QUOTE THESE MESSAGES and they automatically get placed in the Reply Box. Move your cursor where you want to start typing just as you would in a word processing program.
It's awesome! I use it ALL the time.

The QUOTE is great when I am catching up on a thread and there are many things that I want to reply to. I can simply add Quote after Quote, even if you need to read into the next page. They will still be there, ready to be added to the Reply Box when you are ready to reply.