The most important things to remember when planting potted trees:
1) Do not plant too deeply. Most deciduous (leaf) trees are budded or grafted or grown from cuttings. They will have a knob at the bud union. This is normal. Do not bury this knob. It may look odd now, but the knob will gradually dimish as the tree grows. Planting too deeply will eventually kill the tree. Most evergreens won't have a bud union, so plant them at the same depth as in the pot and no deeper. The same goes for seedling trees.
2) Do not disturb the root ball. Years of trials and experience have shown that it really is best to leave the root ball alone when planting. Do not loosen the ball, do not slice the roots with a knife, do not shake off the potting mix and bareroot the tree. These practices are all Bad, especially if the tree is leafed out and actively growing.
3) Do not put anything in the planting hole. Add dirt and water, nothing more. No fertilizer, no compost, no manure, no fish heads, corn meal, nails, chlorox...nothing but dirt and water. Wait a couple of weeks before fertilizing your newly planted trees and don't overdo it.
Plant Me
First, remove the sod. Cut a generous circle where your tree is to be planted and get all that sod out of there. You don't want grass growing right up to the trunk of the tree. Now you can start digging. Measure the size of the root ball and measure the size of your hole as you dig. Don't dig too deeply and don't leave it too shallow. The diameter of the hole should be big enough to make backfilling easy. The sides of the hole should be fairly straight and the bottom of the hole should be flat (not pointed). The root ball of a potted tree has a flat bottom...you want the root ball to sit nicely in contact with the dirt. Once the hole is complete, double check the depth, then place your tree in the hole.
Once the tree is in the hole, at the correct depth, adjust for straightness. Tip the root ball if necessary and put some dirt under the root ball to keep it in place. Backfill the hole about half-way, then step back and check for straightness. Adjust as needed. Once the tree is straight, finish backfilling. Tamp the dirt firmly around the root ball, using the handle of your spade. A little bit of dirt over the top of the root ball is okay, as long as the bud union (the knob) is not covered.
Finally, add a layer of mulch and water thoroughly. Be sure to keep the mulch away from the tree's trunk. Use a hose without a nozzle when watering your tree. Fill the hole with water, let it soak in, fill again, let it soak in, then fill a third time and let it soak in.
Only stake your tree if you absolutely have to.
Give your tree a good soaking at least once a week. Water more often in hot, windy weather.
