If altering a dress seems like too much work, purchase a grey tunic or short dress that fits you and you can skip all that sewing. The dress I found was way too big and baggy, but it was only $2 so I didn't mind putting some effort into fixing it. I'm not a very good seamstress, so if you're looking for good instructions on taking a big baggy dress and turning it into something flattering, there's probably much better instructables for that (like this or this).
First set up a dummy with your dimensions. If you don't already have a dressmaker's dummy, you should try making one from duct tape. Put the dress on the dummy and pin the seams until you are happy with the fit.
I removed the sleeves and shortened the skirt so it would look more like Shredder's tunic.
Carefully remove the dress from the dummy and sew up all your new seams and finish the edges. Try it on to make sure it works and then alter if needed.
Lay out your purple cape fabric. Fold it over and draw out a curve for the top of the cape. You want the cape to be slightly wider than your shoulders. Decide on how long you want the cape and then cut it out. Test out the size and then cut an opening for the neck.
Finish the edges and sew the cape to the dress along the back of the neck and gathered at the shoulders.