I do the tutorial with fat yarn and a size N crochet hook for better visability. It is also a very good idea for beginners to start out with thicker yarn and a larger hook so they can practice before going down to the skinnier threads and crochet hooks.
The hackey sack can be made using the pattern I will provide here, 1 or more balls of #3 crochet thread, and a size 2 crochet hook.
Step 1: Make the slip knot.
Gently tighten the noose around the hook (pic 5).
Step 3: Join with a slip stitch to form a ring.
Insert hook (pics 2 & 3).
Draw up a loop and pull through the chain and the loop on the hook (pics 4 & 5).
Pic 6 is after the hook is through the chain and the loop.
Step 4: Begin crocheting into the ring.
Keeping the tail to the left and on the outer area of ring, insert hook into center of the ring (as pictured in view 1). Draw up a loop and pull it through the ring(pic 3). There are now 2 loops on the hook (pic 4).
Draw up a loop and pull it through both loops on hook (pic 5); one single crochet in the ring complete. Make 5 more single crochet in the ring.
Here is Crocheting a Ball - Part 1 (movie)