Fill a small watering can or bottle with water. Tap water is fine, as long as there is not a high sodium content. If your tap water does have high sodium or other impurities that may be harmful to plants, then use purified water.
Pour the water into the soil surrounding the bonsai. Water the soil until you see the water begin to drip out of the pot's drainage holes.
Check the soil the next day. If it is already dry, you may need to water the bonsai once in the morning and once at night. Keep in mind that, unlike other houseplants, many bonsai roots are extremely porous, so they need to be watered regularly, especially if your home has dry conditions.
Spray the bonsai leaves and greenery with water from a spray bottle once a day. Do this at the same time you water the soil.