Look great! how to you attach the skull to the plaques? just a screw or a hook?

For most....Pre-drill a pilot hole in the base of the skull about an inch forward from spinal hole. Put bar soap on your bit to avoid cracking. Use a coarse 2 1/2" multipurpose screw through the plaque and into the skull. Again, tip the screw with bar soap to avoid cracking. Use slow speed on your drill. Hold skull firmly.
For a removable skull, I use a a 1/4" dowel that goes up into the spinal hole. Most people prefer them just screwed in firm.