First I want to apologize for the horrendous cell phone photos of this project; I did all of the sewing during a particularly slow night at work and didn't take photos of the progress. And to top it off I was in a rush to get it wrapped so I couldn't take the time to lay everything out all nice and pretty like.
I bought the tutu and 3m long red EL (electroluminescent) wire on Amazon, and a square of red felt at Walmart.
I made a pocket for the battery pack out of the felt (I left a little bit of the battery pack sticking out, but it goes all the way in so it's not poking and digging into her back) and sewed it on the inside of the waistband.
Because of the way tutus are made, I ended up using all 3m of the EL wire. I attached it to the bottom of the tutu using an overcast stitch method.
And just for shits and giggles I added a clip on light up arc reactor to the Iron Man shirt I got her.
I really wish I had taken the time to set it up to take better photos, these really don't do the getup justice.