Set the Roots Free
Before placing the tree in the hole, break up the tightly wound root-ball and carefully fan out the roots. Don't pull too hard or the roots will break. It's OK if some of the soil in the root-ball crumbles and falls off. It'll help free the roots. Pulling apart the root-ball encourages the roots to expand into the surrounding soil. If the roots circle the root-ball, but none are thicker than a pencil, use your fingers to tease the root-ball apart.
If the tree is severely root-bound and has circling roots larger than a pencil in diameter, box-cut the root-ball using a pruning saw to shave off all four sides, creating a square root-ball. Once the roots are free, you'll have to be careful when you handle the tree, or what's left of the root-ball will fall apart and you could tear the smaller roots.
Never pick up the tree by its trunk. Instead, support the tree from under or from the side of the root-ball. Set the tree in the center of the hole. Again, keep the root collar about 1 in. above ground level. If it's too high, remove the tree and dig the hole a little deeper. If the trunk flare is too low, add soil under the roots. Cut away all rope, twine, wire, staples and burlap before backfilling (you can leave natural burlap underneath the root-ball if you can't cut it all away).