Use a sharp nail to poke two or three holes in the bottom of a plastic cup. This allows excess water to escape and reduces the risk of root rot.
Fill the cup three-fourths of the way with lightweight potting soil.
Sow one or two mung bean seeds in the cup and cover them with 1 inch of potting soil.
Water until the soil feels moist.
Put a clear plastic bag over the cup and stretch a rubber band over the bag to hold it in place.
Place the cup in a location that receives abundant sunlight.
Add moisture only if the surface of the potting soil dries out completely. Remove the plastic bag after the seedlings emerge.