Thanks! The most important thing is to adjust your lathe properly and to have good measuring devices.
Anyway, after working on the outside of the head I drilled a hole:
Next I had to widen the diameter with a boring tool. I had to adjust the boring bar in order to know how deep I had to go. To adjust it, you carefully get it closer and closer to the part you are working with until it just touches your workpiece without taking material off:
The bigger I made the diameter the more space I will later have for the driver: