Uncover the mosaic and make sure the stones are dry. Scatter topping mix onto the stones, working on a small section at a time, and use a paint brush to spread the mix into the joints between stones. Leave a reveal if you like; it should be no more than one-third of the stones' height.
Tip: The more topping mix you add, the harder the mosaic will set. In high-traffic areas, fill the joints until the mix sits close to or flush with the tops of the stones.