To quote a full post:

Click the button that looks like a quotation mark in the lower left corner of the post you'd like to quote. (It's the button in the middle). The quoted post will be inserted in the reply box. 
You can remove parts of the quote or split it by hitting the "enter" key on your keyboard. 

Reply as desired and hit "save".

To quote a portion of a post:

Either follow the steps outlined above and remove all parts of a post that you don't wish to quote or highlight the desired text. Move the cursor over the highlighted text. "Quote selection" will appear. Click on it. The selected text will be inserted in a quote box inside the reply window. 

Reply as desired and hit "save".

To quote multiple posts:

Click the "plus" sign button next to the quotation mark in the lower left corner in each of the posts you wish to quote. (It's the button on the left). 

A small box will appear in the lower right corner indicating you have quoted X number of posts.

When finished click the "Quote X post" button. All quotes will be inserted inside the reply window with space to reply between the quote boxes.

Reply as desired and hit "save". 

The multi-quote feature can also be used to quote posts from Topic A in Topic B or to quote posts from Topic A in Forum A in Topic B in Forum B. 
To do so, simply click on the "Plus" sign button in each of the posts you wish to quote then go to the topic you wish to reply in.