Dig a square hole instead of round if your soil is clay. 
Water well with a hose the day you plant and for several days water every day. Then water every other day for a week or more and then every 2 days for a while. All along this hose watering install the drip system focusing on the edge of the outside of the original rootball. Use a 24" probe moisture meter to learn what the soil moisture is doing down lower than your finger can test. We like the one sold by Lincoln Irrigation.
Water where the roots are today.... near the outside of the root ball and also one foot outside that for where you want the roots to grow - outward. Roots will also grow down. Amend the soil outward from the trunk and add compost and mulch on the top to feed the tree in the future.