To quote a full post:

Click the button that looks like a quotation mark on the bottom of the post you'd like to quote.

Clicking this button will insert the quoted post within a quote box inside the reply window. Compose the remainder of your post as needed.

To quote a portion of a post:

Highlight the desired text. Then hover the cursor over the desired text. "Quote selection" will appear.

  Click Quote selection. This will insert only the selected text within a quote box inside the reply window. Compose the remainder of your post as needed.

To quote multiple posts:

Click the multi-quote button (a plus-sign and a quotation mark) on the post(s) you would like to quote.

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. This will insert multiple quote boxes inside the reply window. Compose the remainder of your post as needed. You may also use the multiple quote function to save a post from topic A, go to topic B, and quote the saved post in topic B.