To prepare, eliminate all traces of life on your slope. Dig holes where you think the rocks will go . Our Mediterranean dry-summer climate, so challenging to lawns and blowsy English perennials, is actually essential to many rockery plants. And, when you consider that our hillside soils are roughly as stable as Haitian politics, rockeries can be an important insurance against one's property migrating to the neighbors' below. It's much better to cluster them into a few focal points.
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In placing the boulders, experts achieve a natural effect by exposing only parts of each so that they appear to be emerging portions of buried bedrock. In the garden, this translates to lots of low-lying perennial flowers with only a few shrubs and trees. Shrubs and trees are fairly easy. You can achieve a remarkably montane look with a few mountain hemlocks for the vertical framework, with garden-variety heathers, junipers, currants, spiraea and dwarf blueberries around the margins. As for perennials, resist attempting to cultivate a lush western Cascades Delicate Maidenhair Ferns do well in shady crevices.