Cover your work area with a drop cloth. Lay the flats or set pieces on the floor with the side to be painted facing up.
Pour your background color of paint into a roller tray. Gray or light brown are both good background color choices.
Dip your paint roller into the background color, then roll on the rough section of the roller tray. Paint in a zigzag pattern, filling in and painting one flat or set piece at a time. Allow the paint to dry.
Draw the stones onto the flats using chalk. Stones should be roughly uniform in size, but the shapes should vary. Freehand drawing the rocks will keep them looking uneven and natural.
Pour the accent color onto a paper plate. Wad up a plastic bag and dip it lightly into one of the accent colors. One accent color should be lighter than the background color, one should be darker than the background color, and one should be brown or green to evoke dirt or moss. Dab the accent color over the rock. Repeat with each of the other accent colors until you reach the desired effect.
Allow the paint to dry, then touch up the rocks as needed with the background or accent colors. Touch up the “mortar” between the rocks with the background color. Allow the paint to dry completely before using on your set.