Level the ground before you add rainbow rock gravel. Then place weed barrier on the ground. Weed barrier will keep your gravel from getting muddy when it rains. It will also keep your rainbow rock from sinking into the ground. Afterward, add your gravel on top of the weed barrier.
You can also line the edges of your project with steel or polyboard edging. This will keep the gravel from drifting away and will give your project a nice finished appearance.
You may need:
Polyboard or steel edging
Weed barrier staples