Some people are well versed in this, and some tend to do it even when unnecessary, e.g. quoting the last post, when it is just above where they're typing. If it's the previous post, just above, then there's no need to quote it, the topic will flow better without.
Copy and paste instead of quoting is not an adequate alternative, unless you're simply copying a word or two. By not using the quote function, any formatting intended by the original poster will be lost; colours, images, smileys, font sizes etc.
When pressing the button the system will take you to a new screen.
This will include the full text of the post you are quoting. Now includes post_id, user_id, date & time, and a link to take you back to the original post.
Quoting the full post
If you want to keep all of the quoted post then no need to do anything, just add your comments underneath.
Quoting part of the post
Remove the text you don't want to quote. But remember, in order for the quote to work you must leave in the quote tags. These look like [ quote = "some-code-stuff" ] and end in [ / quote ]
Adding to a previous post
If you want to add something to the previous post by quoting, usually a list of names etc. then quote the above post, and remove the two quote tags as shown above. Add what you need to and post. This will post the text without the quoting and make it look like it is your own post and not a quote.
So the result looks like this: