If you already have the walls and you want to plant them then you must remove some of the soil from the crevices with a widger or teaspoon before planting. Check if there is an adequate amount of compost in the crevices to support the plants. Use seedlings or small rooted cuttings, since these fit easily in the crevices.
Place the plants on the flat of a stone and using a widger, small dibber or a pencil to ease the roots into the crevices. Do not try to cram roots into too small spaces as this will damage the plants. Bed the plants down into compost and add more compost into the crevices to hold the plants in position. Firm the compost well with a widger or with your fingers, if enough space, to remove any air pockets. It may be helpful to wedge small stones around the collars of the plants to hold them in place and to prevent the compost from becoming dislodged.
When you finished planting all the plants, water them from the top of the wall or with a mist spray and keep spraying them regularly until the plants are established. After several days top up any sunken areas with spare compost and periodically check the plants and refirm any that have worked loose.
When planting in vertical crevices, fill the crevice with a gritty compost and carefully ease in the roots of the plant. Cover the roots with compost and then wedge in a small stone, sloping it down the rock-face. Firm further compost into the crevice.
When planting in walls you should use a special compost mix for filling the planting crevices in the walls. The right mixture is made of 3 parts loam or good sterilized garden soil, 2 parts coarse peat substitute or peat and 1-2 parts sharp sand or grit. Use some extra grit, sand or stone chippings for plants that are to be grown in the wall itself to ensure a good drainage.