1. Go to any post (doesn't matter how old it was).  Remember that this site's stats show that it gets literally hundreds of new visitors daily (from internet seaches) and many of those go back and several days back and some actually read every post. They most often don't post themselves, but they read it all.  So adding to any post provides information that gets seen.
2. To make your addition (or response) just click on the little "cartoon buble" icon. 
3.  You will be given a comment input form for you to make your addition.
4.  After you hit the submit button, your response post appears under the original post. 
5. There is no limit to how many response posts can be made post (or number of reponses your addition). It can create quite a conversation thread. 

1.  Use the add to a post (response) feature to make additions to your own posts.
2.  Sometimes we want to say some more about what we just posted.
3.  Sometimes we need to make a correction about some facts that we stated wrongly (or explain about a typo). 
4. Just use the same technique as adding to any post (click the ) to add to your own posts.

It is not a problem for you to create a new one line post (make a new post with the "Write a comment" link), just to announce that you have added to a previous post which has scrolled on down the page (or now on another page).  In fact, you can  include a direct link to that older post (just use the link capture feature  which is explained in the "Sharing posts" help).