Sketch a diagram of the keyboard on a piece of paper. Lay out the intended dimensions for width, length and height.
Choose a smooth, flat stone that fits the dimensions of your sketch. Make sure to leave room for letters and dates you might want to add later. Different types of stones have different properties for carving. For example, limestone might be easier to work with, but a stone made of granite allows for sharper lines because of its hardness. You can determine the hardness of the stone using the Mohs hardness scale.
Polish the headstone with a buffer or another appropriate tool.
Mark out the diagram onto the headstone. Redraw your original sketch directly on the stone or make a life-size drawing. If you choose to make a life-size drawing, cover the back of the drawing with charcoal, set the charcoal-covered side on the headstone and retrace the drawing with a pencil. The charcoal should transfer to the stone, giving you a stencil in which to work. Darken the stencil with a grease pencil or paint marker so you can see the lines better. Tighten up your lines with a ruler or straightedge. Make sure all the lines are straight and all the keys are the same length and width.
Use the tooth (pointed) chisel to outline the drawing and etch it into the stone. Make the perimeter lines around the keys that correspond to the white keys 1/4-inch thick and 1/4-inch deep. You will remove the stone from the keys that correspond to the black keys so the perimeter-line thickness is not as important. Keep all your cuts smooth and consistent.
Use the flat-head chisels to remove stone from each of the the "black keys." Remove 1/2 inch of stone from each black key. Make sure to chisel out smooth, flat rectangles.
Use a grinder equipped with a fine-toothed diamond-tip disk to smooth out all of the marks. Equip the grinder with an adjustable depth gauge to ensure all cuts are the same depth.
Blow all of the stone dust from the etching with an air hose and compressor. The stone should now resemble a piano with outlined white keys and hollowed-out black keys.
Paint the perimeter of the white keys and the insides of the black keys with black, waterproof acrylic-enamel-based paint.
Cover the stone with a waterproof lacquer.