Here are my abbreviated instructions for transplanting plants with a root ball (not bare root transplants).

From my experience adding bare root plants I maintain the original root ball whenever I can.

First prepare the plant to consolidate it's root system by defining the size of the root ball.

I do this in fall when a plant has lost its leaves.

Based on the size of the plant, decide the likely size of the root ball.

Push with a sharp shovel a circle around the base of the plant to the depth of a shovel. This severs roots that have extended away from the base. It encourages the plant to manage with a reduced root ball and defines the size of the hole to be prepared.

Prepare a hole about 50% wider than the size of the root ball defined in pruning. I guess how deep the hole needs to be

Filter out rocks and debris.

Move the plant to its new home just before it begins to return from dormancy in the spring.

Dig up the root ball, which has consolidated as a result of the root pruning. Capture as much of the root system that is within the defined circle.

Add subsoil from the hole. Adding top soil collected or imported.

I add amendments such as potassium and phosphorous, but not nitrogen. Sometimes I add root stimulator if I have disturbed the root ball.

Regular watering for the first year at least, to boost each through the summer droughts.

For detailed explanations on transplanting trees and shrubs see: