Stamp down the soil in the bottom of the hole with your foot, so it is firm and won't sink when the tree has been planted. Place the root ball into the hole. Check to see that the top of the root ball is at—or a little higher than—ground level. Use a utility knife to cut any twine wrapped around the trunk and fold down the burlap around the sides of the root ball. Don't remove the burlap completely as this can damage the root ball. The tree's roots will grow through the burlap into the surrounding soil and the burlap will eventually deteriorate.
Fill in the hole around the sides of the root ball and pat it firmly around the ball. Form a ring of soil around the bottom of the tree that will allow water to pool there like a basin. This will help keep the roots well-watered until the tree is established. Add 2" to 3" of mulch to combat weeds, retain moisture and insulate the tree from temperature extremes. Be careful not to cover the trunk; keep a no-mulch ring of 1” to 2” around it.
If your tree is bare-rooted, prune any damaged or unhealthy-looking roots (healthy roots should have abundant root hairs). Then build a cone-shaped mound of dirt in the center of your hole and set the tree on it so the trunk flare (where the roots spread at the base of the tree) is visible and the tree's crown is 2" above the top of the hole. This allows for settling to occur naturally. Fill in the hole and pack the loose dirt around the trunk.
It's a good idea to mix some peat moss and a small amount of plant food into the soil you replace around your new tree.
Some trees may need to be staked in place to avoid damage from the wind or to simply be held in place while settling occurs. On each side of the tree, hammer standard wood stakes into the ground with a mallet and tie a length of string from the stake to about halfway up the tree. Tie it firmly but give it a little room to flex in the wind.