I'm about to grout my cross with the blue Mosaic pieces, modeled after the cross with the tan and brown pieces shown. As with the tan cross, I have left about a quarter of an inch for a grout line all around the edges of the top surface of the cross. I don't know how to grout this, or how to keep the grout from falling off the edges of the Cross and making a mess of the side edges. How is this done?