When planting a rock garden on a steep slope there isn't much one can do to safely go about tilling or preparing the soil. Usually these areas are well-drained and do not require preparation anyway. Choose smaller rocks and boulders that you and your friends and family can manage, or have a professional landscaper place larger ones for you.
Planting 
After boulders and rocks are in place you can go ahead with planting plants you have purchased from your local nursery and garden center, or online mail-order nursery.
TIP: Use the Advanced Plant Search to find any or all of the plants listed on this site that are suitable for use in rock gardens. When you get to the Plant Search, simply select your USDA Zone, then choose "Rock Garden" under USAGE TIPS.
On steep slopes it is best to integrate groundcovers, such as creeping sedums and ice plants. These evergreen perennial succulents will spread throughout the garden area and between other plants and shrubs. Groundcovers will prevent erosion and also replace the need for mulch.
Space larger growing shrubs a good distance apart and use the smaller plants and groundcovers between. Try to avoid overcrowding plants. Rock gardens look best when each plant can be noticed. Avoid planting too many of the same plant. Try to make every plant in the garden a unique specimen.
When installing plants on a slope simply remove them from their containers and plant them so that the top of the root-ball is level with the existing grade. Using an organic soil ammendment mixed with sand to add to the soil you remove from digging the hole. Dig holes twice to three times as wide as the container the plant came in. Tamp soil mixture as you backfill to remove any air pockets.
Level Site Rock Garden
If you know that the soil in a more level site is rocky, sandy, and well-drained you can follow the same instructions above for planting a rock garden on a slope.
If you know or suspect that the site you have selected for your rock garden does not drain well, then there will be a little more soil preparation involved before you can begin planting.
The best thing you could do is to raise the bed area up with sandy native topsoil, then turning in additional sand and gravel to enhance drainage. Raise the bed up by 10" or more above existing grade if possible.
If raising the bed is not possible you will need to do the following:
Once a site is selected and its boundaries are marked off, the area should be dug out to a depth of at least a foot or more especially in low ground. One condition that rock garden plants plants almost invariably require is good drainage. The excavation should then be refilled with a 50/50 mixture of the soil removed from the excavation and gravel, rock, broken brick, or other stone material. After backfilling the mixture, soak the bed down so that soil will fill any air pockets and then backfill with more of the soil/gravel mixture as is necessary to fill.
You are now ready to plant. Remember not to overcrowd larger plants in the rock garden. Select as many different types of plants as possible including groundcovers. Rock gardens should be interesting to look at.
