Sand the surface of the bare shower walls using an electric sander with a fine-grit sanding belt. That task will remove debris and roughness from the surface.
Vacuum the shower walls and surrounding areas with a shop vacuum cleaner.
Spread a 1-inch thick layer of medium grout over the shower walls with a trowel.
Press 1/2-inch thick, flat river rocks halfway into the grout, spacing the rocks 1/4 inch apart from each other.
Spread a 1/4-inch thick layer of medium grout over the rocks. Scrape off excess grout with the trowel.
Wipe off the surface of each river rock with a damp washcloth. Allow the river rock walls to air-dry for 24 to 36 hours.
Provide ventilation for the shower area. Apply a layer of waterproof, polyurethane sealant over each entire river rock wall using a 3-inch wide paintbrush. Continue to ventilate the area well while you apply the sealant. Allow the sealant to dry for eight hours.