Starting in one corner, apply a layer of thinset across the top of the concrete base. Gently push the first river rock mesh square into the thinset. Continue setting thinset and laying tile squares until the surface is completely covered. After all the tiles have set up for four to six hours, mix grout and apply with a float. Push down firmly to ensure that the grout gets in between all of the individual pebbles. Immediately wash off excess grout with a damp sponge. Have a bucket of clean water handy to rinse the sponge frequently.