To water a Christmas tree, it's important to keep the soil moist but not soaked. A good rule of thumb is to check the soil every day and water the tree when the top inch or two feels dry. Water the tree until the water starts to drain out of the bottom of the stand. You should also check the tree's water level every week and add more water if needed. Additionally, it's a good idea to mist the tree with water every few days to help keep it hydrated.