First of all, you should buy a balloon from a nearby party shop and fill it with helium gas, that is quite common and you can get it from the party shop as well.
Now you need to find the midway point along with the dowel. Use measuring tape for this purpose and made a mark at the 18-inch point.
You should calculate two midway points which should be between the ends and the centre. On a dowel which is 3 foot long, both of these points will be at equal distance, which will be 9 inches, from the ends of the balloon.
The next step you should follow is that take three inflated balloons and attach them to dowel by using a tape. Place two balloons at both ends of the dowel and the third one to the centre.
You should attach the tape to one side of cup and then give the cup a position at the midway point between balloons. Now you need to fold the tape over the dowel to the other side of cup. Similar process should be repeated with the second cup.
You can also add a few pennies into each cup in order to keep the balance of the ballast and it will stop the balloon from floating away. Make sure you have added equal number of pennies in both cups in order to create perfect balance.
Keep adding pennies until the balloon gently blimp. When the balloon starts floating gently in a stationary position, then stop adding pennies.
While adding pennies, make sure you have gripped the balloon firmly otherwise it can float away from you.