I will do a piece about that in spring, but a quick description comes here.
If you have a flat rock or something similar, you can use the same method as when arranging a forest, or a single tree. Using Keto, that is a black sticky clay soil to build a barrier around, so the soil doesn’t wash off.
Be sure to mount screws or drilling holes at the bottom, so you can fix the plant in there with wire.
A second method is to build a base like when arranging a Kokodama. Kokodama is a decorative simple arrangement where a plant is planted in a moss-ball arrangement. Also here the Keto bæsticky clay soil is used, and afterwards covered by mosses, attached with a thin cord or sewing thread.
Set this Kokodama arrangement at top of the piece you have selected.
A video with the processes and how-to will be published in spring.
Link to a dealer with Keto. https://www.kaizenbonsai.com/keto-tsuchi-japanese-peat-muck-kokedama-bonsai-soil-1kg
If you google bonsai keto soil it will come up.