To prune begonias, start by removing dead or diseased leaves. Then, use sharp shears or a saw to shape the plant and encourage new growth. Cut back long stems to a desired length, and thin out crowded areas to improve air circulation. After pruning, dispose of cuttings safely and care for the plant by providing adequate water and light. If necessary, apply rooting hormone to cuttings to promote root growth. Watch for over-pruning and slow recovery, and adjust your pruning techniques as needed.