Step 1: Materials list
12" round balloons
The pattern is available at the USA Volleyball site above. It'll tell you how much to enlarge the pattern pieces. (I'm showing only one end piece, since they're the same size--the only difference is that you'll put a buttonhole in one.) The ruler shots indicate the dimensions I used, in case you trust yourself to make your own pattern. Mine accommodates a 12" balloon very nicely.
By the way, do read the information on the USA Volleyball site. They won't tell you this, but it's kind of like a teddy bear for people who probably don't need to be seen in public with teddy bears--the texture is comforting, it's a little cuddly, and if you keep the balloon good and firm, you can bounce it off somebody's head without doing too much damage.