This website offers a free pattern and tutorial on how to make a Harry Potter Wizard Robe. This robe looks really realistic and you can adjust the pattern to fit a child or adult. It is perfect to wear as a fancy dress costume or at a Harry Potter Themed Party. I would say it you would probably need some sewing experience for this project. The reason being, you would need to print out and assemble the pattern and if your not used to this, it could be a bit tricky. However, the sewing itself looks pretty straightforward. Follow the link for the for the pattern and tutorial; http://www.clairesanders.net/2011/10/wizard-robe.html
If you would like to learn or brush up on other basic sewing skills and techniques before you start this project go here; Sewing Techniques & Stitches to find the skill you would like to work on.
If you would like to see easy/ beginner sewing projects go here; Easy/Beginner Projects
If you would like to see more challenging/Experienced projects sewing go here; Challenging/Experienced Projects