Water the paperwhite narcissus as you did during the blooming season – enough water to keep the soil slightly moist without rotting the bulb. Keep it in the same light as when it was in bloom. This helps the bulb build up the nutrients it spent producing the blooms.
Wait until the foliage begins to brown to cut back on watering. Do this gradually, allowing the top inch of soil to dry before watering, then the top 2 inches, until you aren't watering at all. This forces the paperwhite narcissus bulb to go into dormancy.
Use small pruning shears or sharp scissors to cut the foliage to 1 inch in height when it dies back. Place the entire pot in an area that remains dark and dry.
Dig up the bulb in the fall. If it is soft or mushy, throw it away. Otherwise, plant it in a pot of fresh, well-drained potting mix, water until the soil is moist and leave it somewhere warm.