Measure the height of the upright piano's string box from the top to the floor. Then measure the width of the box and the keyboard. Finally, measure the depth of the piano from the front of the keyboard to the back of the string box.
Multiply the width of the keyboard by the depth of the piano. Multiply the height of the string box by four. Add these two answers together to get the area of the cover you will need. Buy enough muslin to cover that area and place it over the piano.
Use your fabric pen to draw the piano's outline on the muslin, including the top and sides of the string box, the curves out from the string box to the keyboard, and the bottom of the front of the piano. Cut along the lines you've made with your shears.
Lay your cover material facedown. Set the muslin cutouts on top of it. Draw lines around the cutouts on the material and cut the pieces out of the material. Connect all of the pieces with straight pins, then use your sewing machine to connect all of the pieces. If you sew the pieces covering the sides of the keyboard and string box to the middle pieces, and then sew those pieces to the large piece in the middle, you'll have enough room to turn your material over and use your cover.