you open the last message... the click on "reply to message"
That will open another typing window box with the last message ( which you can either chose to erase and replace with your own new text reply...or leave it there and just add your new text reply above it or below it) and then you type your reply text. when finished with your text:
Then make sure you re-enter your name in the upper box "name" field and then click on "submit follow up"