This part is tricky, but do you best to draw lines, in the same direction, with equal spacing, for your keys.
Connect the top right corner to the cutout's corner. Draw another set of 3 lines to create the area the keys will be in. From the top of the squares, draw a cutout using 3 lines. I tried to learn them to draw a simple and plain piano diagram with key-names (A, B, C, C#) written on the keys, but the code is nested with other requirements so much that I couldn't make any sense of it. Draw a thin line between the squares, along the bottom. Using the piano roll in Garageband, like other DAWs, is one of the most useful features of using any digital audio interface, mostly because you can draw. This and this answer provide a beautiful code to draw a piano with it's range.
