A wide but quite shallow planting hole and site the plant so that a good portion of the rootball is above the existing grade. Break up the clay that was dug out and use that to backfill, creating a small mound or berm. If you need more soil to finish the mound, you can use bagged soil, compost, mulch or other amendments but do not mix in....just as a finishing layer or topdressing. And bring the soil mound only to the edge of the top of the rootball, not over it.

A clay soil is not necessarily poorly draining :-) You can test by doing a perk test (instructions online).

Check out this thread for a recent detailed discussion: https://www.houzz.com/discussions/5914612/soil-ok-for-these-trees#25509314

Thank you so much!