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.
Step 2: Cut it out
Step 3: Sew the side pieces
The second picture here shows what it'll look like when you've sewn all but the last seam.
The third picture shows how far on both ends to sew those first and last panels together--about 3". There will be a space down the middle of those two panels to make it possible to sew in the end pieces.
Stitch the seams flat around the end openings. This will make it a lot easier to stitch through the end pieces and avoid bunching of fabric.