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Step 1: Get Your Stuff & Start!
-Hacky sack (duh :P)
-One of those light up bouncy balls like these.
-Sewing needle and some thread
First, you want to get the little plastic ball out of all that rubber surrounding it. You can do this with an Exacto knife of sorts or something like that. A pic of it is shown below.
Step 2: Cut an Opening, Sew, & Finish
After that, put the little ball in, sew it up, and you're good to go! Have fun hacking it up in the dark or whatever pleases you! ;)