How to make your own Balloon Drop
How many balloons do you plan to drop?
You need to know about how many balloons you'll have before you can purchase the net. Mylar balloons are all about the same size. Latex balloons are a bit smaller. You'll probably want custom balloons either way.
Get the netting:
It can be bird netting or strawberry netting or fish pond netting from a landscape supply store. Buy the netting in standard sizes or have it cut to the size you'd like. Figure you can fit 250, 11" balloons in a net that's 17' x 15' and 600, 11" balloons in a 17' x 30' net.
Stuff the net with the balloons:
Spread the net out on the floor and begin to "stuff" the net with the inflated balloon. To do that, you'll need to make a sort of bag out of the net by doing a weaving stitch through the short edge to make the bottom of the pocket and then down the length of the netting as you fill it up. Be careful NOT to tie off or secure the farthest end of the line running down the length of the netting. It has to be able to pull completely free later. (When you're all done, you're going to flip it over and attach the unsewn side to the ceiling, leaving the stitched side hanging down. When the time is right and the cord is pulled, the stitches running the length of the net will pull free and the balloons will drop!)
Hang the filled netting from the ceiling and let the pull rope dangle out of sight until needed:
Use ceiling clips or c-clamps. Even cup hooks will work in a pinch. Just be sure they're up there to stay for the drop. Hook the unsewn side of the netting to the clamps. (The point is to have the netting open and drop the balloons in the process, not to have the net fall down from the ceiling.) Make sure the net is high enough off the floor so people aren't lunging at it before the drop. Also be sure the pull rope is out of sight - for the same reason.
When the time is right:
Pull the pull rope in a steady motion. The line should come out in an easy motion. As it does, the net will un-sew and the balloons will drop.