Prepare the thin-set by mixing it with water if that's required for your particular product. Smooth the thin-set all over the brick surround with a flat finishing trowel until it is level and even everywhere. Wait 24 hours.
Create a cardboard template of the fireplace surround. Plan out the design for the tiles on top of the template. Determine which tiles need to be cut; outline the cuts backward on the backs of the tiles with a pencil, leaving room for grout lines.
Put on safety glasses. Perform the necessary tile cuts with a diamond-bladed wet saw. Arrange the tiles on the template again and make sure you are happy with their appearance.
Build a wooden bolster out of 2-inch by 4-inch boards that is 1 inch to 2 inches longer than the firebox on either side and approximately ½-inch higher. Set the bolster just in front of the opening of the fireplace with the top even with the bottom edge of the firebox (the horizontal top portion of the surround). Make sure the bolster is centered in place.
Spread a new layer of thin-set over the firebox. Make this layer thinner than the last but just as even. Comb through the thin-set with the edge of a notched trowel, making the lines as straight as possible.
Lay the stone tiles in place, beginning at the center bottom of the firebox and moving up. Finish the center of the firebox, and then alternate laying the side tiles. Let the thin-set harden for two hours and remove the wooden bolster.
Mortar and tile the rest of the fireplace surround. Lay the tiles from the bottom to the top. Keep the tiles as straight and level as possible.
Wait 24 to 48 hours for the thin-set to dry completely. Remove any loose pieces of thin-set from between the tiles. Cover each tile with strips of painter's tape.
Grout the tiles in sections that you can finish before the grout dries. Scoop up the grout with a grout float and drag it all over the surface of the surround, working the grout down between the tiles. Scrape up any extra grout with the edge of the float and wipe away stray bits on the tiles with a dampened sponge; wait at least 24 hours before using the fireplace.