The way I do it, Angel, it to right mouse click on the Quote button in all the posts I want to quote, except the last one. On the right click, I select, 'Open In New Window'. When I have the additional windows opened, I then just do a regular left mouse click on the Quote button of the last post I want to quote. That opens a window for you to post in, with the quote there. I then go back to the other opened windows, highlight the text that it in them, including the quote tags and brackets, right mouse click, copy, come back to the main post you are working on and paste the copied text. Do that with each opened window. Post your comments, generally after each quoted text, close the extra windows, and submit.
With that much work, it is always best to highlight and copy your finished product, as the DIS is not the most stable platform and does bomb out often enough, in which case you would lose all your work. Best is to highlight your post, and copy it, then submit. If the DIS bombs out, at least you have your post in memory.