This hairstyle is from the movie cover of Barbie in a Mermaid Tale 2. This tutorial is for an everyday realistic hairstyle, but if you are going for it to look just like the cover, there are a few changes that I'll tell about at the bottom of the post.
regular french braid first. This will be like that, but you only add hair to one side. When you add the hair in, gather a new section all the way back to the part, and cross it over the middle strand.
You can put it into a side ponytail for a variation.