Fill a container with commercial potting soil. Dampen the potting soil.
Cut a stem from a healthy 'Green leaf white' begonia. Pinch off leaves from the lower half of the stem.
Make a hole in the potting soil, using a stick or pencil.
Dip the bottom 1 inch of the stem in powdered rooting hormone. Plant the stem in the hole, and then pat the soil lightly around the stem.
Cover the pot with a clear plastic bag. Place the pot in bright, indirect sunlight, where the begonia stem is protected from hot, direct sunlight. Normal room temperatures are appropriate.
Water as needed to keep the soil lightly moist. Never water excessively, or the begonia stem may rot.
Remove the plastic bag when the begonia shows healthy new growth. At this point, water the potting soil only when the top 1 inch feels dry to the touch.
Allow the begonia to mature indoors for three or four more weeks. At that time, leave the begonia in the pot and move it outdoors, or plant it in a flower bed. Be sure outdoor temperatures are above freezing before moving the plant outdoors.