Step 6: Make countersinks
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.