An old-fashioned hose on a slow drip is one option. Move it around so that the soil is moist under the entire canopy of the tree. Alternatively, a water bag at the base of a small or young tree allows for a quick fill and slow release of water over an extended period of time, which encourages deep watering.
How much water your tree should receive depends on the tree size. A general rule of thumb is to use approximately 10 gallons of water per inch diameter for each watering. Using a hose at medium pressure will take approximately 5 minutes to produce 10 gallons of water. General formula for watering: Tree Diameter x 5 minutes = Total Watering Time.
Small Trees (1-7" diameter) - Water once a week
Medium Trees (8-15" diameter) - three times a month
Large Trees (16+ diameter) - three times a month