Place rice ball on a lightly greased piece of banana leaf at least 2 inches square (or larger, as the ball will expand when cooked). Use scissors to cut the leaf into the size you want, and vegetable oil to lightly grease it. You can also use squares of greased parchment paper or wax paper.
Place the rice ball (with its "mat") in a covered container. OR place directly inside a bamboo steamer (or other type of steamer) - with the lid on, but off the heat.
Continue making rice balls until all the dough has been used up.