Refill with amended soil and add the plant, when finished the soil should be at the same level it was in the pot. If the plant is in a pot, remove it. If it is in burlap, first position it, add some soil to hold it in place, then untie and fold the burlap away from the plant, finish filling being sure all the burlap is covered with soil. Most plants benefit from a topping of mulch.
Several inches is plenty.
Trees should be staked. Use a Mouse Guard on all your newly planted hardwood trees and shrubs to protect from mouse damage. Mice like the bark of many plants and they can easily cause a lot of damage. Save yourself a lot of trouble, put Mouse Guards on all your trees and ornamental shrubs. Make them large enough to leave in place for several years.