Too much mulch will prevent water and air from reaching your plant. Don't let the mulch touch the trunk of the tree - it can damage the bark and lead to disease. Finally, water well. The other planting method is to not dig a hole, but make a raised bed around the roots of your new plant. I haven't done this with trees, and I don't think it would work very well.
Tree roots spread more than shrub roots and eventually they'd reach the edge of the raised bed.
This might damage the tree. But I've done it with bushes and it works fine. Put the root right on the ground, and then add compost over the roots to the depth you would normally plant the roots. This compost will settle, so you need to check the level occasionally over the next few months and make sure the roots aren't exposed. Then mulch like normal and voila you have an instant raised bed area for your bush.