I did a flat iron tutorial with shorter hair and you can check it out here...Click Link. This video is how to create loose curls with longer hair.
In the flat iron tutorial, I tell you to hold the iron tightly and slowly pull the hair in the iron. If you do this, you create beautiful ringlets. If you want more loose curls at the bottom, I do the same thing, BUT when you get to the ends of your hair, pull the flat iron through quickly so it doesn't have enough time to create a tight curl.
Below is the video to create loose curls with a flat iron.