Length of cord needed is determined by how long you want the dragonfly to be. The formula for cord needed is (length of dragonfly body) x (10). For example, if you wanted the dragonfly to be 4 inches, then the total length of cord to cut off would be 40 inches of cord. For a 3-inch, total length would be 30 inches and so on.
This can also be made without the barrette and makes a nice little key chain fob. I included this pic below. Also, it can be made with multiple colors. You just have to fuse the colors together at some point.
This is a series of loose and tight cobra knots. I did not include the tutorial for the cobra. You can either look that up or try and figure it out from the pics here.