The tricky part is working out the wing loops and of course getting the tension right to hold the form. This comes with practice. A more elaborate version is found in the book Chinese Knotting by Lydia Chen. It can be an interesting camp craft for idle time to keep young hands busy. The exact amount of material required depends of course on the material used but you can start with about 2 feet if you are using common twine.
Rule of thumb is more length for larger material and less for smaller material.
Too much is always better than to little in this case. Yang and Ming Jyh-Chen of Taipei. Yang adds beads for eyes at step 2 to her version. Let the ends fall as shown leaving the center bight to the length of the dragon fly that you are making. This will be the length of the body.