You will need a Pennant Builder Punch and a 3/4" Circle Punch. Punch out the straight edge triangle from the Pennant Punch and mark a centre-point on all sides. Punch the circle and trim a crescent shape from the bottom. See pic below for guidance.
Now glue the cap top onto the circle and your grad cap will look like this!
I usually add it to the card on a dimensional for more visual effect. If you add wings to your owl, he looks like he's tossing the cap into the air. Of course, this cap would look great with a tassel. I couldn't find one small enough and couldn't make one to look good, so I left it blank. Either way, it's a great way to add a cap to your owl.