I made this awhile ago for Famous day during school. I decided to be a superhero and superhero's need capes. Not really need but it's a distinct feature. I put it together rather fast and it was super easy. Here you go!
First get your pillow case out! If it is old you might get a weird faded cape.
Next take your seam ripper and take out all of the seams of your pillow case. I know this step will feel like forever long.
Next lay it out and see which sides are the shortest. Take one of the short sides and fold it in by about a half inch depending on the size of your ribbon.
Sew down the half inch side making sure to leave sides open to run your ribbon though. When the ribbon is inside your tunnel (What a teacher of mine described it as) you are done! Vwala a simple cape for the superhero inside of you.