The style of bonsai in a rock is said to be in Ishitzuki style, with plant roots that sink into the soil, or with the whole plant clinging to the rock. Before attempting to prepare a bonsai of this type we have to choose a decorative rock, which will marry harmoniously with the plant that we have decided to grow in this style, and possibly full of cavities, which we can eventually fill with soil. The plant to be grown in this style must have part of the root already exposed above the ground, and be well lignified. If you don’t have any bonsai that exhibit these attributes, you can create one over a few months, by taking your plant and periodically removing one or two centimetres of soil from the pot, in order to uncover the roots. You can temporarily put small rocks in the cavity that is formed between the stem and the ground, between the roots. When the plant is showing quite a lot of its root system you can move it to the stone you have chosen.
Put the stone in the pot and remove the bonsai from the soil, washing the residue of earth from the roots. Then support the plant with the rock, and fix the part of the root system that will be exposed on the rock. You can use raffia or thin bonsai wire for this purpose, taking care not to overtighten the roots, and not to damage them irreparably with pieces of wire. Then cover the part of the root system that will be covered over with bonsai soil. If you are preparing a bonsai that will have its roots in the rock then you have to place the entire root system in a cavity of the rock itself, so it is important to find a suitable rock, which has one or more large cavities, and also a suitable bonsai, whose roots are quite compact, and can be inserted in a narrow space. Also in this case you can use the same raffia or wire to fix the roots to the rock, and also the special moist and sticky soil, which will help you to "cement" the plant into the cavity of the rock.