There's another related instructable: LED Floaties by tetranitrate. The floaties didn't get to float up up and away, but made a cool temporary art installation at a park!
Video of the construction and launch:
Step 1: LED Throwie minus magnet
There is also some maths you can do to make sure your balloons are going to fly:
The volume of a sphere:
V = 4/3(pi)r3
And we used 12" latex balloons, so r = 6 inches, which gives ~904 cubic inches or ~15 liters of helium per balloon.
You can approximate for every liter of helium used, you can get about 1 gram of lift, the battery and LED were about 4 grams, the balloon itself was around 3 grams and we can give another gram for the tape and URL paper. The payload for the helium was a total of ~8 grams, which is a ready for launch Houston!
Step 2: Stuff your balloons
The video in the intro has a good demonstration on how to do this, but basically just have someone stretch the neck of the balloon like it was a sock about to be put on and have another person insert the LED unit. Simple!
Step 3: Fill them up and bag 'em!
You can start bagging them for transport, we just used large black lawn bags, which held around 7-10 balloons each. In total we had about 70 balloons, including one large 24" with 5 LEDs on it!
Step 4: Release!
(*but not too guilty, I talked to her about the project and she said "Don't worry about it! I'm just a rock in outer space hosting some very silly self conscious life forms... the one's who will waste time and energy making comments about the impact to their water table have actually done more damage to me themselves at the end of the day!" and I was like "Cool, thanks Mom! I love you.")