Summarize:

The longer sides of the rectangle should be 6 in (152 mm) in front of the rubber and 24 in (610 mm) behind the rubber. The shorter sides should be 18 in (457 mm) from each side of the rubber. Use a pick and a shovel to remove the existing soil to a depth of about 6 in (152 mm). Take care not to disturb the pitcher’s plate. Add a single layer of clay bricks designed for pitcher’s mounds to provide durability to the plateau and reduce the need for frequent maintenance. You can find them at specialty sports stores and online.  Use a tamp to ensure the bricks will stay in place.
Mark a 5 ft (1.5 m) by 3 ft (0.92 m) rectangle around the pitcher’s plate. Dig out the existing soil inside the rectangle. Place clay bricks in the rectangle and tamp them down.