For my Billiard Ball costume, I took a pattern for a tennis ball. Any pattern for a round object will do. I cut out the pattern in black felt (since magic 8 balls are black). I then traced a round plastic lid onto white felt and cut out.
I printed out an 8 from my computer and with the left over black felt cut out and eight. I sewed the 8 onto the white circle (you can glue on). I sewed the circle onto the back of the costume using invisible thread and a close zigzag stitch.
I then cut a similar circle in the front of the costume. I made a triangle with white foam and printed magic 8 ball sayings (nope, try again, and maybe). I tied and glued invisible string to one of the corners of the triangle. I then used a food saver bag and sealed one end with the string hanging out so it would look as if it was floating.
I blew air into the bag and sealed the other end. You have to be quick or you will loose air. I then hand sewed a pouch to slide the bag into the costume.
I followed the pattern for the hat using black felt again. I stuffed the costume with newspaper and pillow stuffing.