-Packing tape (not pictured)
First cut the bottom of your plastic cup. If your cup cracks a bit, like mine did, I would just reinforce it with some packing tape. Next tie a not at the end of your balloon, and cut off about a 1/2 inch from the other end.
Next, using two hands, stretch the balloon over the bottomless cup. Try to have the knotted part of the balloon directly in the middle.
Then finally, add some packing tape around the balloon to prevent it from popping off the cup.
Now put a few marshmallows inside the cup, pull the knotted end of the balloon and let go to shoot.
I have high ceilings in my house, so the boys were trying to see if they could get all the marshmallows to touch the ceiling.
Next time we play this, we'll have to go outside and see who can shoot it the furthest distance.