Dig a hole in the ground where you want to plant your tree. It should be the same depth as the root ball and at least twice as wide.
Standing downhill of the hole, looking uphill, dig slightly farther into the soil on the hole’s far side. This helps the hole retain water.
From this same position, build a soil mound on top of the grass just outside the hole’s bottom rim. This prevents runoff and helps the ground retain water flowing downhill.
Plant the tree and refill the whole with a 50/50 planting soil mix. Scatter mulch around the tree’s base and over the soil hump you created.