just click on Reply , which is on the bottom of every comment that you want to reply to.
if you are adding a youtube link, or any other link for a video you need to copy and paste that link in the comment section.
if you are adding a link from camera or computer as a video or image
you need to click on the choose file, and select the image from your camera or computer and load it there.
Then fill up your name, email address, and click on post comment.
