The best site for a rock garden is sloping or terraced, ideally with southeast exposure. It should be completely free of deep — rooted weeds. If no such space is available, build up a raised bed with southeast exposure. The alpine plants traditionally used in rock gardens require soil with no clay and with excellent drainage. If the existing soil has clay, remove any ground cover and excavate the site to a depth of around 18". Replace the soil with equal parts loam, peat moss, and coarse sand.
Begin at the base of the site, placing the most substantial stones first. The idea is to create the impression of a natural subterranean rock formation weathered from exposure. Set the stones in the soil so they are at least half buried and so their most weathered surfaces are exposed. Slightly less than half of the surface of the site should be rock. Avoid even spacing or rows of rocks. Once the rocks are in place, cover the soil with a mulch of complementary pea gravel or rock chips.
"New" rocks moved to the site will need to weather before they will look natural. Encourage weathering by promoting moss and lichen growth. In a blender, combine a handful of moss with a cup of buttermilk or yogurt. Distribute the mixture on the exposed faces of rocks with a putty knife to promote moss growth.
After the rocks are in place for a few days and the soil has settled, begin planting the garden. As with the rocks, focus on native plant varieties and strive for natural placement, without excessive variety. You can plant several sizes, from small trees or shrubs to delicate alpine flowers. Place plants in crevices and niches between rocks, allowing them to cascade over the surface of the rock.