Process
A steam-filled balloon must have a source of water, a heat source to boil the water and a channel connecting the steam to the balloon itself. Depending on the size of the balloon, place a tube of water underneath the open opening of the balloon. Then attach a burner underneath to boil the water, which evaporates to form steam. Make sure the channel to balloon opening is not blocked.
Sustainability
Due to the fact that the steam is rising, it would not exit the balloon from the bottom opening. In addition, the burner would help the inside of the balloon maintain its super hot environment and thus stay afloat. The water molecules inside will remain as steam until the air is cooled down by extinguishing of the burner.
Increased Flying Time
Steam balloons can extend their flying time almost without limit. As the steam in the balloon cools it will return to water form. A separate tube can be used to collect the water and channel it back into a container. The container is then placed under the burner to produce steam and repeat the process. However, there is usually some inefficiency with the collection process so the water will eventually be depleted. The fuel for the burner will eventually expire as well.
Bottled Balloon Project
One simple steam balloon project uses a plastic bottle. For this project, fill the bottle a third or quarter of the way up with water. Tape or clamp a balloon on top of the bottle opening so that it is sealed air tight. Place the bottle on top of a source of heat until the water is boiling and steam is created. The balloon will then lift off.