Make sure that you put the light and weed blocking tarp material down on the ground before you place your rocks. Then place the rocks in any way you want.
You'll want to remove any grass or weeds from the area, spread out a weed block fabric and pour on your chosen river rocks. Edging is a good idea to help retain the rocks and make a definitive separation from you lawn. They will help keep unwanted weeds down and retain moisture.