Make sure the field tile has dried if you recently installed it. Dry set the border, making all of the cuts beforehand to ensure that the setting process goes smoothly and quickly. The border for this project consists of a 4-inch decorative border on paper backing composed of glass and 1x3 slate tiles and a back border created from the 12x12 slate field tiles.
Follow up by dry-setting the decorative border. To dry-set the decorative border, find the center of each border section, mark that line with a straightedge and a pencil and set the decorative border from the center of that section to the nearest angle. This step ensures that your decorative border will have even cuts on either side of each decorative border section.
With the border completely dry set, mix the thinset and begin setting the border tile.
Set the back portions of the tile border first (Image 1). If the decorative border being used is 4 inches wide (with joint size accounted for), lay a wood 2x4 in the area where the decorative border would go so that the back border will have a straight edge to run up against. Once each section of the back border is set, remove the 2x4 and run a level or square from the field to the back border to make sure the areas' levels are matching (Image 2).
With the back portion of the border set, set the decorative border (Image 1). Apply and notch the thinset with a small margin trowel, place the decorative border in the thinset and press down with the rubber float to ensure that the border is setting in the thinset. Because the decorative border used in this project had a paper backing, once the thinset for the decorative border has hardened, wet the paper backing and peel it back. In the waiting spaces, set small one inch glass squares using thinset (Image 2).
After the thinset completely dries, thoroughly clean the tile floor and seal it. Use a color enhancing penetrating sealer designed for stone material. Sealer is a product where you get what you pay for, so don’t be afraid to pay a little more because it will help you in the end. Apply the sealer following the manufacturer’s instructions, but use clean white rags because colored rags could bleed color onto the slate.