Set two 24-inch boards so they're parallel, 21 inches apart and set two 21-inch boards perpendicularly between them so their sides are flush with the ends of the 21-inch boards. Lay the cement board on top of the boards so the edges are all flush and screw the cement board to the boards. Use three screws for each board.
Cut the ends of the 25 1/2-inch boards at a 45-degree angle so when laying flat on their longest side they have a trapezoid shape. Position the short side of each board against the 2-by-2-inch boards and the edges of the cement board so the corners are flush. Position the boards so their top edges are ¼ inch above the cement board. Screw two screws through each of these boards and into the 2-by-2-inch boards. These boards will hold your tiles in place.
Set the tiles on the cement board so the lay flat. You can use this marble inlay over any counter-top or mount it to a stand to be used as an end table.