NO amending!! - What comes out of the planting hole is all that goes back in. If planting in heavy or poorly draining clay soils, dig a very wide (at least 2x, 3x is better) but quite shallow planting hole with rough sides. Place the rootball in position with top above grade 1-2". Remove any ties completely and the burlap from around the trunk. Mound excavated soil up to top of rootball and to fill in the sides of the hole. If necessary, break up the clay soil before refilling.

Any amendments you may think you need should be used as a topdressing only.

gardengal is correct, but I would add one bit to it. I have very clay soil, and almost always plant on a mound so that about half of the rootball lies above the original grade. This is to provide drainage until the roots extend into the surrounding clay at the level they need to be. This has worked VERY well for me.

Arktrees

Yes, when in doubt, plant high :-)) It is always preferable to plant anywhere above grade than it is to plant too deeply. One can even plant with the entire rootball above grade if other considerations are properly attended to.