The process for planting hostas is not much more different than any other plant. The planting hole should be dug at least a foot deep. The width of the hole should be one and a half times the expected mature size of the clump. Check with the nursery or web site where you purchased your hosta for the plants expected mature size.
Generally, hosta roots grow and spread horizontally, so a large wide hole is best. When planting hostas that are in a container, carefully remove the plant from the container. Sometimes the roots may be bound to the container. Tapping the container sides should loosen the roots from the pot. If the roots are difficult to loosen, it may be necessary to cut through some of them, make several vertical cuts in the routable. Shake the excess soil from the roots and do not use potting soil in the planting hole.