Now take another piece of sand paper and sand down the edges of the holes. Then take a countersink cutter and make the holes wider for the head of the screws. If you don't do this, either the screw will not sink completely into the surface or the wood could crack.
Sand down the edges another time after that.
Now take the front part and mark the holes for the screws. I suggest to use fewer screws than on the back side. The top half of the front must be loose to be able to swing and produce a good sound. I suggest to use 2 screws on the long side, the first 6cm from the edge and the second 8cm from the first. And 4 screws on the short side, 3cm from the edge and 8cm from each other. That looks good.